Bontril vs Phentermine

November 20, 2015 at 5:32 PM

Are you overweight? Have you tried many weight loss programs without any avail? So if you want to lose weight quickly then you need to read this article as it will tell you about two weight loss drugs known as bontril (phendimetrazine) and phentermine. We compared both drugs for you to help you understand how bontril and phentermine work.


  1. Bontril
    1. Legal use
    2. Medical uses of Bontril
    3. Dosage and intake methods of Bontril
    4. Mechanism of action
    5. Actions after intake – pharmacokinetics
    6. Bontril overdose
    7. Side effects
    8. Decreased efficiency causes
  2. Phentermine
    1. Medical uses
    2. Dosage and intake methods
    3. Mechanism of action
    4. Pharmacokinetics
    5. Overdose
  3. Clinical studies for Bontril vs Phentermine
  4. Conclusion


The scientific name for this drug is phendimetrazine. Bontril is a stimulant drug which belongs to the morphine chemical class. It is used as an appetite suppressant. This drug acts as a prodrug of phenmetrazine and almost 30% of any dose converts into it. This drug is an extended release and less abusable version of phenmetrazine. Phenmetrazine functions as a norepinephrine-dopamine releasing agent (NDRA) (1).

The main structure of Bontril includes methamphetamine which is a CNS stimulant. When a methyl group is added to the structure of amphetamine the overall potency and bioavailability of the drug is increased. But the added methyl group sometimes results in making the compound almost inactive until the methyl group is removed from the metabolism of the body. This makes a steady activation of the drug in the body which lowers the abuse of the drug and allows about once a time daily.

Bontril is potent as amphetamine and comes under Schedule III of the Convention on Psychotropic substances. Bontril is a Schedule III controlled substance in the United States under the Uniform Controlled Substances act of 1970.

The list of psychotropic substances under international control lists Bontril as a Schedule IV controlled substance drug. Bontril is also present under the Schedule III substance under the U.S. Controlled substance act of 2007.

Medical uses of Bontril

Bontril is a phenyl alkylamine sympathomimetic amine having the clinical pharmacology quite similar to other prototype drugs of this class, which are known as the amphetamines. Some major actions of this drug include the elevation of blood pressure and stimulation of the central nervous system. The drugs of this class are known to show tolerance (2).

Bontril is commonly used for the treatment of obesity; it is, therefore, called an anorectic drug. But it isn’t yet known that whether the main function of this a headache is appetite suppression or something else completely.

Other metabolic effects and nervous system actions may also be included in suppressing the overall appetite of a person. It has been seen through clinical trials that adults who are given anorexic drugs like Bontril and diet management instructions lose more weight as compared to placebo.

One thing is important to mention here and it is that the magnitude of weight loss of anorexic drugs over placebo is relatively quite small (about 1 pound per week).

The rate of weight loss is greatest during the first few weeks and the ratio of weight loss decreases in the succeeding weeks. The origin of various drugs due to the increase of weight loss hasn’t been established so far. The amount of weight loss due to anorectic drugs varies according to the conditions of the trial.

It has also been seen that the weight loss is more related to the variables involved than the prescribed drugs; these variables include the population treated, the physician’s expertise and the particular diet given. There is currently no study done regarding the relative importance of the drug.

Obesity is measured in several years but the clinical studies are currently restricted to a few weeks’ duration. So the total impact of weight loss on diet alone is considered quite limited.

People with weight-related medical problems can also take this medication. Some other problems which are cured by Bontril include:

Dosage and intake methods of Bontril

The specific dosage of Bontril has been mentioned below:

  • The usual dosage of Bontril is about 1 tablet of 35 mg to be taken twice or three times a day one hour before meals. The dose should be given according to the individual needs of the patient in response to the lowest effect of dosage. Sometimes the half tablet per dose (17.5) may be appropriate. The usual dosage shouldn’t exceed 2 tablets three times a day.
  • Bontril slow release capsules of 105 mg should be taken about 30 to 60 minutes before the morning meal. The Bontril capsules aren’t recommended for children under the age of 12 years.
  • The renal dose of Bontril isn’t fixed.
  • The liver dose of Bontril is not fixed either.

Tolerance to the drug usually occurs within a few weeks. When the tolerance occurs then you shouldn’t exceed the drug in an attempt to increase the overall effect; you should discontinue the drug.

Intake methods

The following are the intake methods of Bontril:

  • The immediate release tablet form of this drug should be taken through the mouth about 2 to 3 times a day about 1 hour before or after meals or according to the instructions of the doctor. You shouldn’t take this drug late in the day because it can cause sleeping troubles.
  • The sustained release form of this tablet should be taken once a day about 30 to 60 minutes before the morning meal. You should swallow the medication whole. Don’t chew, crush or break the capsules. Doing so can create serious troubles and long-term side effects.

The dose of this drug is based on your medical condition and response to therapy. Leave it to the doctor to adjust the best dose for you. You should take this medicine daily or exactly prescribed by the doctor to get the most benefits. You should take the medication at the same day every day.

You should not here that this drug should only be taken for a few weeks’ time and shouldn’t be taken with other appetite suppressants. The risk of side effects increases if this drug is taken with other diet suppressants.

The drug can also cause certain withdrawal symptoms if you are using it for a long period of time or in high doses. Many withdrawal symptoms can occur in such cases. You should discontinue the drug in these cases. You should reduce the dose gradually to prevent any withdrawal symptoms (3). You should consult your pharmacist or the doctor immediately if you experience any withdrawal symptoms of the drug.

The medication may also stop having much effect if you are using it for a long period of time. Talk to your doctor if this happens. Don’t increase the dose by yourself. The doctor may direct you to stop the medicine.

Mechanism of action

Bontril is a white and odorless crystalline powder. It dissolves in warm water and also dissolves in warm alcohol but it is insoluble in acetone, benzene, ether and chloroform. This drug has similar pharmacological activity as compared to other drugs of morphine class, which reduces obesity.

When Bontril enters the body then a part of it enters the body and metabolizes into phenmetrazine which may stimulate the central nervous system and releases serotonin by targeting the hypothalamus part of the brain. The elevated levels of the neurotransmitter in the body result in high quality of blood glucose and the release of norepinephrine and epinephrine.

These two neurotransmitters are responsible for glucose release from the liver and its transmission to muscle cells. This causes an increase in the rate of metabolism, suppresses the feeling of hunger and speeds up the weight loss process.

The rate of the heart and the blood flow is increased due to the energy which gets dissociated after the burning of fat.

Actions after intake – pharmacokinetics

The following steps are involved in the pharmacokinetics of Bontril (4):

1. Absorption

Bontril is absorbed directly from the gastrointestinal tract through the oral route. The action of duration is approximately four hours after release. The action duration in sustained form is longer than that.

2. Distribution

Currently, there is no data with regards to the volume of distribution, protein binding or the plasma clearance process of Bontril.

3. Metabolism

Bontril is metabolized by an N-oxide product and a demethylated product.

4. Elimination

The main route of excretion of Bontril is through the kidneys where most of the metabolites are excreted. A study reported that approximately 30% of the unchanged drug was found in the urine when a single dose of Bontril was given.

Currently, there isn’t any data of pharmacokinetics of Bontril in patients with renal disease. There is also no data on the hemodialysis of Bontril.

Bontril overdose

Bontril overdose can happen just like any other drug. Some major signs and symptoms of an acute overdose of Bontril have been mentioned below:

  • Confusion
  • Restlessness
  • Panic states
  • Hallucinations
  • Belligerence

The central stimulation is usually followed by fatigue and depression. Some cardiovascular effects include vomiting, diarrhea, abdominal cramps or nausea. The poisoning can lead to coma, convulsions or death.

The overdose management of Bontril is mostly symptomatic. Bontril is sedated with a barbiturate. The use of rapid-acting acting alpha receptor-blocking agent should be considered if hypertension is marked. The experience of hemodialysis isn’t enough to recommend the use of this drug (5).

Side effects

Bontril belongs to the Schedule III of controlled substances. This means that this drug can easily turn into an addiction. It is related chemically to the amphetamines. Amphetamines and other related drugs are now illegal because they have been extensively abused in the past.

The abuse of Bontril is related to intense psychological dependence and social dysfunction. There have been reports of patients who have increased their dose to many times than the recommended dosage.

This drug should only be used for a short term period because it can cause psychosis which can lead to schizophrenia. You should stop taking this medication quickly after the administration of high dosage when you suffer from extreme fatigue and depression.

Serious side effects

One major side effect of this drug is that tolerance is sometimes developed due to this drug. Tolerance to the drug usually occurs within a few weeks. You should discontinue the drug when this happens; don’t try to increase the dosage to get the desired effect.

Some other major side effects of this drug include difficulty sleeping, dry mouth, irritability, vomiting, diarrhea, nausea, vomiting and constipation. You should immediately inform the doctor if these side effects worsen.

You should remember one thing that the doctor has given this medicine to you because he thinks that its benefits are greater than its side effects. Most of the people using this drug don’t suffer from any serious side effects.

You should also inform the doctor if you suffer from any serious side effects like mental mood changes, change in sexual ability, irregular heartbeat or uncontrolled muscle movement.

You should also stop taking this drug if rare side effects occur like a headache, seizure, vision changes and weakness on one side of the body and vision changes.

Sometimes serious side effects can be caused by the use of this drug like lung or heart problems which include heart valve problems and pulmonary hypertension.

The risks increase with larger doses of the drug and the use of this drug with other appetite suppressant drugs. You should stop taking this drug if you notice any unlikely and serious side effect. You should inform your doctor immediately if you suffer from any of the following side effects:

  • Difficulty breathing with exercise
  • Chest pain
  • Fainting
  • Swelling of legs and ankles
  • Decreased ability to exercise

Sometimes an allergic reaction to the drug can also be caused but it is quite rare. You should immediately inform the doctor if you suffer from any serious side effect like rash, swelling, dizziness, itchiness and breathing troubles.

Some major nervous side effects of the use of Bontril include restlessness, dizziness, overstimulation, headache and tremors. Sometimes a person may suffer from mental episodes like flushing, swelling and agitation.

The drug may also cause some genitourinary side effects, such as libido decrease or dysuria.

Bontril may also cause some ocular side effects, such as blurriness of vision.

Moderate side effects

Some moderate side effects of Bontril are shown below:

  • Anxiety
  • Decreased ability to exercise
  • Fainting
  • Feeling warmth
  • Passing urine more often
  • Redness of face
  • Swelling in hands and feet
  • Trouble thinking clearly
  • General weakness
  • Pain or discomfort in neck, arms or back

You should inform the doctor immediately if you suffer from any of these side effects. People living in the USA can report the side effects at 1-800-FDA-1088 and the people living in Canada should report the side effects at 1-866-234-2345.

Decreased efficiency causes

The following things reduce the efficiency of Bontril:

  • The efficiency of Bontril is decreased when it reacts with other drugs. Bontril may interact with 459 different drugs. Some common medications which can react with Bontril include Celebrex, Metformin, Nexium, Synthroid, tramadol, vitamin D3, Lyrica or Hydrochlorothiazide.
  • You should inform the doctor if you have any kind of allergies. There are certain products present in this drug which reduce the efficiency if the drug is taken by allergic people.


Phentermine is a contraction of “phenyl-tertiary-butylamine”. It is an anorectic drug of the substituted amphetamine and phenethylamine chemical classes and has a pharmacology similar to amphetamine. It is used by people as an appetite suppressant.

This drug has been approved for use as an appetite suppressant in Australia and is given as a diet pill. It is mostly used by obese people for whom overweight is considered as a medical risk. Phentermine helps in weight loss and is used for short-term use along with a healthy diet and exercise.

Phentermine has been approved by the FDA and is being studied at present along with other drugs for the treatment of obesity. One such combination made by researchers is phentermine/topiramate. This combination has been approved for sale in the United States since 2012.

Phentermine is available in different forms like tablets, capsules or drinks like Adipex, Vites or Qsymia. There is a different dose in each of these forms.

In general, the active ingredient (phenyl-tertiary-butylamine) content in Phentermine pills or capsules is 37.5 mg.

Medical uses

Phentermine is used for a short period of time for people who are overweight (6). This drug is given when constant exercise and diet aren’t producing the desired result. Phentermine belongs to a class of drugs which are called anorectics. These drugs work by decreasing your appetite.

Phentermine initiates the release of brain chemicals which manipulate a person’s mind to curb the appetite so that he feels full and eats less. The caloric intake of a person is reduced when there is a reduction in the absorption of things like starch and fats and they don’t get metabolized.

You should remember here that this drug should be used with proper exercise and a weight loss diet program. You should consider the diet pill as an addition to your weight loss program.

A person may suffer from some side effects of phentermine during the initial intake of the drug because the body isn’t adapted to this drug. Some general side effects which may occur are digestive issues, insomnia or anxiety. But these side effects are usually not serious and disappear after some time.

Your main goal to get rid of obesity when taking phentermine should be to bring your weight to a normal level. Patients with I and II degrees of obesity lose weight easily as compared to the patients with III and IV levels of obesity. So you should use the phentermine tablets without waiting for the time when severe obesity occurs. Obesity can also create certain other health issues in which phentermine is contraindicated like high blood pressure and changes in a heartbeat.

Many cases have been reported in medical practice in which the patient lost 20-30 kg weight by the intake of phentermine with proper exercise and diet. A significant weight loss can’t be achieved until there is a desire in the patient to shed the extra pounds. Most fat people who use phentermine lose about 5-15% of their original body weight.

People who take phentermine are more motivated when they see the visual results. So they should keep a diary with them in which they note their weight loss.

Some other health issues which can be treated by the intake of phentermine have been mentioned below:

  • Diabetes
  • High blood pressure
  • Shorter life expectancy
  • Heart diseases

Dosage and intake methods

The dose of phentermine is different from patient to patient. You should follow the instructions of your doctor carefully or read the instructions on the label. The information given here only relates to the average dosage of this drug. You shouldn’t change your dosage if the doctor has recommended a different dose for you.

The dosage amount mainly depends upon the strength of the medicine. Some other factors which influence the dosage amount are the number of doses you take each day, the length of time you take the medicine and the time between the doses.

The following is the proper dosage of this drug:

  • Capsules – 15 to 30 mg in adults and teenager from 17 years or older. It should be taken approximately 2 hours after breakfast. These capsules aren’t recommended in children younger than 16 years.
  • Tablets – One tablet should be taken each morning by adults and teenagers of 17 years or older. The usage of this drug isn’t recommended for children younger than the age of 16 years.
  • Extended-release capsules – Adults and teenagers older than 16 years should take one capsule a day before breakfast or 10 to 14 hours before sleeping. Children shouldn’t use these capsules.

You should take the dose as soon as possible in case you miss a dose. But you should skip the dose if it’s time for your next dose. Never double a dose.

Intake methods

You should take this medicine exactly as instructed by the doctor. You shouldn’t take the medicine more often or longer than that recommended by the doctor. If taken for long-time the drug may cause an addiction.

This drug is available in four different forms: Capsules, disintegrating tablets, extended released capsules and tablets. The intake methods of these forms is mentioned below:

  • Extended-release capsule: Swallow extended capsule whole. Don’t break, crush or chew it.
  • Disintegrating tablet: You should take this tablet with or without food. You should make sure your hands are dry before touching the tablet. Place the tablet on the tip of your tongue. It will melt quickly. Take it with a glass of water when it melts.

You should follow the instructions of the doctor carefully along with a proper diet plan and regular exercise. You should talk to your doctor before starting an exercise program.

You should take the last dose about 4 to 6 hours before bedtime to avoid any kind of sleeping troubles.

Mechanism of action

There is some similarity between phentermine and its parent compound known as amphetamine because they are both TAAR1 agonists in which the activation of TAAR1 in the monoamines help in the releasing of the neurochemicals into the synapse. Phentermine acts as a releasing agent of norepinephrine in the neurons when the doses are quite relevant. But it releases dopamine and serotonin into the synapses too (7).

The release of monoamine from VMAT2 is also triggered by phentermine. Phentermine decreases a person’s obesity by the reduction in the hunger perception which is mediated through different nuclei present within the hypothalamus. Phentermine releases norepinephrine and epinephrine outside of the brain which causes the breakdown of fats and the fat cells.


The following steps are involved in the pharmacokinetics of phentermine (8, 9, 10):

1. Absorption

The maximum plasma concentration in phentermine (Cmax), time to Cmax (Tmax) comes under the concentration curve from time zero to last measurable concentration time and the area which is under concentration curve from zero to infinity. The phentermine pharmacokinetics aren’t affected by higher doses of the drug. When the phentermine 15/100 mg dose is given then the mean concentrations ratios for AUC and Cmax are about 2.5.

2. Distribution

Phentermine is about 17.5% plasma bound. The estimated phentermine volume is about 348 VL through the pharmacokinetics population analysis.

3. Metabolism

There are two metabolic pathways of phentermine which are p-hydroxylation on the aromatic ring and N-oxidation on the aliphatic side chain. Phentermine is commonly metabolized by cytochrome P450 but it doesn’t show any kind of extensive metabolism. Phentermine isn’t metabolized by monoamine oxidase.

4. Elimination

Approximately 40% of the dosage exists as unchanged in the urine when administered alone. The mean half-life of phentermine is about 20 hours. The estimated phentermine oral clearance is about 8.79 L/h through the population pharmacokinetics analysis.


A person can overdose on phentermine just like any other medicine. Phentermine overdose happens when a person intentionally or accidentally takes more than the recommended dose of the drug. You should seek emergency help at once if you think that you have overdosed on phentermine. You can call on poison help line at 1-800-222-1222. An overdose of Phentermine can become fatal.

Some common symptoms of Phentermine overdose are shown below (11):

  • Hallucinations
  • Panic
  • Feeling aggressive
  • Confusion
  • Diarrhea
  • Vomiting
  • Irregular heartbeat
  • Irregular breathing
  • Seizures
  • Fainting
  • Feeling light-headedness
  • Overactive reflexes

Phentermine side effects

Some major side effects of phentermine include dizziness, difficulty breathing, irritability, nausea, vomiting, constipation or diarrhea. You should inform the pharmacist at once if these side effects occur to you.

You should remember that the doctor has prescribed this medication for you because he thinks that it has more health benefits for you.

Phentermine can increase your blood pressure. You should inform the doctor immediately if there is a spike in your blood pressure.

You should also inform the doctor if any of the following serious side effects occur:

  • Irregular heartbeat
  • Sudden mood changes
  • Uncontrolled muscle movement
  • Change in sexual ability/interest
  • Seizures
  • Headaches
  • Slurred speech
  • Vision changes
  • Weakness on one side of the body

The use of Phentermine is also related to an elevation of serum enzymes but there are no studies to prove that phentermine can cause acute liver injury (12).

Sometimes an allergic reaction can also be caused by this drug. You should inform the doctor at once if you sense any symptoms of a serious allergic reaction like dizziness, trouble breathing, allergic reaction, rashes, itching or swelling.

You can call the FDA at 1-800-FDA-1088 to get medical advice about your side effects.

Decreased efficiency causes

The following things reduce the overall efficiency of phentermine:

  • The efficiency of phentermine is significantly reduced when it reacts with other drugs. About 460 different drugs can react with phentermine. Some common drugs which can react with phentermine are Ambien, Prozac, Singulair, Vitamin D3, Vitamin B12, Lyrica, Nexium, Xanax or Zyrtec).
  • Diabetic patients should avoid the intake of Phentermine because the efficiency of the drug is reduced when diabetes patients take it.

Clinical studies for Bontril vs Phentermine

The following research provides a brief comparison of Bontril vs Phentermine:

  • Phentermine was approved in a 56 weeks trial in which the patients were randomized to placebo or one or two doses of phentermine. The period of the study was about 1 year. The study results showed that patients who continued their study regimen for an addition 52 weeks lost about 9.3% and 10.5% of their body weight compared to only 1.8% weight loss in the placebo group. The main problem with phentermine were its adverse effects. The adverse effects of phentermine were more than the placebo group. Some common side effects included dry mouth, constipation or insomnia.
  • Another study took place recently in which more than 3000 female patients over the age of 35 years were studied. There was no experimental data of the studies. The study was done based on the reports from the user reviews. Different information about the patients was gathered, like their age, gender, weight loss etc. through the social media and other tools. The results showed that both the drugs were more or less equal in their weight loss effects but it was found that Bontril had fewer side effects as compared to phentermine. So the researchers selected Bontril as a better drug out of the two because of lesser side effects.


The results from the above studies clearly show that Bontril is a better drug out the two. The main reason for this is due to lesser side effects of Bontril. Phentermine has more side effects as compared to Bontril. So you should choose Bontril over phentermine whenever you think of losing weight through drugs.

This article does not replace a proper medical advice. Before taking any medicine always ask your doctor.
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Written by: Michal Vilímovský (EN)
Education: Physician
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Published: November 20, 2015 at 5:32 PM
Last updated: February 8, 2017 at 4:15 PM
Next scheduled update: February 8, 2019 at 4:15 PM
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