Phentermine: A Detailed Guide + Resources

What is Phentermine?

Phentermine is a drug prescribed as an appetite suppressant to treat obesity, along with exercise and a reduced-calorie diet.[i]  This means you also have to change your diet and exercise regularly while taking Phentermine. Phentermine is prescribed for short-term only (lasting less than 4 weeks).

You need a prescription to purchase Phentermine.

Phentermine suppresses appetite, and it does so by causing changes in the central nervous system. Phentermine is often prescribed to treat patients with obesity that failed to lose weight using other methods. Your doctor will determine if you can have treatment with Phentermine.[ii]

Phentermine is somewhat chemically similar to Amphetamine, thus use carries some risk of addiction.[iii] Doctors will tell you this risk before prescribing the drug. The risk of getting addicted to Phentermine increases if you took more than recommended by your doctor.[iv]

Phentermine is a restricted drug. Selling or giving away Phentermine is against the law and could cause harm to others.[v]

History of Phentermine

Phentermine was first approved by the U.S Food and Drug Administration on May 4, 1959, as Ionamin.[vi]  It became popular because it costs less than its competitors Xenical and Meridia.[vii]

Phentermine in combination with other drugs was later introduced on the market. Phentermine and Fenfluramine (known as ‘FenPhen’) were approved in 1973 to treat obesity. Dexfen-Phen, another formulation consisting of Dexfenfluramine and Phentermine, were also approved as an appetite suppressant.

However, studies suggested that ‘Fen-Phen’ and ‘Dexfen-Phen’ are associated with heart valve disease, so the FDA withdrew both drugs from the market and warned the public about its dangers.[viii] Therefore, you cannot purchase Fen-Phen and Dexfen-Phen in pharmacies.

Phentermine remains approved and available on the market. A new formulation called ‘Phen-Pro’ combines the use of Phentermine capsules or tablets with the drug Prozac, an antidepressant. The addition of Prozac may help prolong the effect of Phentermine.[ix] Prozac and Phentermine are both approved in the market and available on a prescription-only basis.

Brands of Phentermine and their differences

In the United States, Phentermine is available in generic (phentermine) and branded versions.[x]

The brands Adipex-P, Oby-Cap, T-Diet, and Zantryl, have Phentermine hydrochloride as the active ingredient and are available in tablets and capsules.[xi]

Other brands of Phentermine include Suprenza, Lomaira, and Qysmia. Suprenza delivers Phentermine in an orally-disintegrating tablet and can be taken with or without water.[xii] Lomaira contains 8 mg Phentermine hydrochloride per tablet and is used when smaller doses are needed.[xiii] Qysmia contains Phentermine and the anti-seizure drug Topiramate and is also used approved to treat obesity. The addition of Topiramate enhances the effect of Phentermine.[xiv]

What you should know before taking Phentermine?

You must tell the doctor your medical history before taking Phentermine. Medical history consists of health information such as pre-existing health conditions, drugs taken, history of surgery or other medical procedures, and use of alcohol and restricted substances.

You cannot take Phentermine if you are pregnant or nursing a child. Losing weight during pregnancy can cause problems to the unborn baby. Phentermine itself is proven to cause abnormalities to the fetus.[xv] If you are overweight and pregnant, stick to your doctor’s advice.

If you have previous health problems, tell them to your doctor right away before being prescribed with Phentermine. Treatment with Phentermine is not safe if you have the following medical conditions:[xvi]

  • Any history of heart disease
  • Uncontrolled high blood pressure
  • Thyroid diseases
  • Abnormal heart rhythm or arrhythmias
  • Glaucoma
  • History of mental disturbances such as agitation or paranoia
  • Pregnant or breastfeeding
  • History of alcohol abuse or drug abuse
  • Experienced allergies to Phentermine, weight loss pills, amphetamines, stimulant drugs, or medicines used to treat colds.

You need to be closely monitored by your doctor if you have the following conditions:

  • Diabetes
  • Hypertension
  • Kidney disease
  • Any problem affecting the thyroid
  • Allergy to the drug Aspirin or to yellow food dye (FD & C Yellow No. 5, or tartrazine)

You cannot take Phentermine if you are also taking other medicines containing Phentermine, like Phen-Fen. Doing so greatly increases the risk of having pulmonary hypertension. Do not take other weight loss medications with Phentermine without doctor’s advice.

Phentermine has some habit forming potential. Never give Phentermine to other people. Always keep Phentermine in a secured place where others cannot access them.

Note that Phentermine may impair your senses and reaction times, which can affect things like driving or operating heavy machinery. Do not perform important or dangerous tasks soon after taking Phentermine.

How to take Phentermine?

Phentermine is an oral drug, so it is taken by mouth. Your doctor will determine your dose of Phentermine. Doctors usually start at the lowest dose and increase it until effective dose is reached. Thus, you must always take the dose as prescribed and stick to scheduled clinic appointments.

Phentermine is available in 37.5 mg, 30 mg, or 8 mg tablet or capsule. The usual dose of Phentermine is ½ to 1 tablet once a day, while 8 mg pills are taken 3 times a day. Some Phentermine tablets are scored so they can be split.

Most Phentermine brands are taken on an empty stomach, 1 hour before or 2 hours after breakfast.[xvii]  Suprenza can be taken with or without food. Phentermine should be taken in the daytime. Taking Phentermine near or at night can result in insomnia.

What happens if I miss a dose?

If you missed a dose of Phentermine, take it as soon as you remember. If it is nighttime or if it almost time for the next dose, skip the missed dose and return to the original dosing schedule.

Do not take an extra dose of Phentermine to make up for the missed dose. Doing so can result in an overdose.

What happens if I overdose on Phentermine?

Overdose can happen if you take more than the prescribed dosage. The symptoms of Phentermine overdose include:

  • Profound restlessness.
  • Rapid breathing.
  • Rapid heart rate.
  • Hyperactive reflexes.
  • Irritability and assaultive behavior.
  • Panic attacks.
  • Nausea and vomiting.
  • Abdominal cramps and diarrhea.
  • Irregular heart rhythm.
  • Too high or too low blood pressure.
  • Fatigue and depression, after the effect of the drug wears off.

What should I avoid while taking Phentermine?

You must avoid taking weight loss drugs or supplements while you are taking Phentermine unless the doctor tells otherwise.

Your doctor will advise you not to drink alcohol while taking Phentermine. Alcohol may increase the risk of adverse reaction.

Avoid taking stimulants and illicit substances such as cocaine, heroin, coke, or cannabis while you are taking Phentermine.

Side Effects of Phentermine

Like any other drug, Phentermine may cause side effects.[xviii] Here are the following side effects that are serious and require immediate medical attention:

  • Very high blood pressure symptoms such as a severe headache, blurred vision, buzzing in the ears anxiety, chest pain, shortness of breath, irregular heartbeats, and seizure.
  • Shortness of breath, even with mild exertion.
  • Chest pain.
  • Feeling of passing out.
  • Swollen feet or ankles.
  • Pounding or fluttering heartbeats.
  • Confusion or irritability.
  • Extreme happiness or sadness.

Phentermine may also cause an allergic reaction to some people, which requires prompt attention and the person must be brought to the hospital right away. Here are the symptoms of an allergic reaction:

  • Feeling or sensation of tightness in the chest.
  • Swelling of the face, tongue, or throat.

Phentermine may cause minor side effects that can be bothersome. The minor side effects of Phentermine include:

  • Reduced libido or impotence
  • Diarrhea or constipation
  • Upset stomach
  • Insomnia
  • Dryness of the mouth, or unpleasant taste
  • Headache
  • Dizziness or tremors
  • Restlessness or hyperactivity

Common drug interactions

Phentermine may interact with certain drugs and cause undesirable effects. To avoid drug interactions, make sure to tell the doctor all the medicines you are taking before starting treatment with Phentermine.

Here are the common drug interactions of Phentermine[xix]:

  • Alcohol
  • Antidepressants
  • Drugs used to treat colds, stuffy nose, cough, and flu
  • Stimulants, including Amphetamine and Methamphetamine
  • Appetite suppressant drugs
  • Methylene blue
  • Ma Huang
  • Contrast agents or dyes used in X-ray, CT scans, or other imaging procedures

Pregnancy Warnings

Phentermine is highly contraindicated in pregnant women. Phentermine is an FDA pregnancy category X drug, and animal studies showed it may cause birth defects.[xx] Therefore, Phentermine should not be taken at any time during pregnancy.

The safety of Phentermine to newborns and young children is not known. Do not take Phentermine if you are nursing, as the drug can be excreted into the breast milk.

Dosage of Phentermine

The dose of Phentermine is highly individualized. Your doctor will start at the lowest dose and increase it gradually until the effective dose has been reached[xxi].

The usual doses of Phentermine for obesity are:

  • 24 mg per day (three 8 mg tablets or capsules)
  • 15 mg per day (1/2 tablet)
  • 5 mg per day (1 tablet)

In case dose adjustments are necessary:

  • 12 mg per day (three 4 mg tablets or capsules)
  • 75 mg per day (1/2 tablet)
  • 5 mg per day (1 tablet)

Tips to make the most of Phentermine

Always follow the doctor’s prescribed dose. Do not stop or change your dose of Phentermine your doctor’s advice.

For best results with Phentermine, you must also restrict calorie intake, stick to healthy diet practices, and exercise regularly.

Phentermine is good for short-term only and using it longer than recommended puts you at the risk of addiction and other health problems.

Using other stimulants can add to the stimulating effects of Phentermine, and that could be dangerous to your heart. Do not take diet pills, supplements, or other stimulants, unless ordered by your doctor.

If you are taking Phentermine, make sure to keep up with clinic appointments and talk to your doctor. It is the only way to determine the progress of treatment. It is also the opportunity for your doctor to examine you and adjust your dose if necessary.

Stopping Phentermine

Your doctor will determine the right time on stopping treatment with Phentermine. Treatment may be stopped if the drug achieved its intended effect, have caused severe side effects, or if it is not working.

The usual practice is to taper the dose of Phentermine for several days, and this should be done with doctor’s supervision.[xxii] Abrupt cessation of Phentermine may trigger uncomfortable withdrawal symptoms. If you want to stop taking Phentermine, be safe by calling your doctor first.


Q. Can I get Phentermine as an OTC?

  1. No. You need to have a prescription to purchase Phentermine. Purchasing the drug without a prescription is against the law.

Q. Phentermine is what type of drug?

  1. Phentermine is an anorectic drug, and it works by suppressing appetite.

Q. Can I use Phentermine as a stimulant?

  1. No. Using Phentermine without doctor’s supervision can result in addiction or serious heart or blood vessels problems.

Q. Does Phentermine show up as positive on drug test?

  1. Because it has similarities to Amphetamine, persons taking Phentermine may have false-positive drug test results. However, it will not show up as positive for Amphetamine use.[xxiii] In that case, tell them you are taking Phentermine.

Q. How long does Phentermine stay in the system?

  1. Phentermine reaches peak levels in the body in 3 to 4 hours after taking it. Phentermine has a half-life of 19 to 24 hours so it may take 2 to 3 days for the body to eliminate much of it in the system.[xxiv]

Q. What should I do if I am not losing weight with Phentermine?

  1. Go to your doctor right away. You may need dose adjustments or have to undergo an examination to determine other health problems.

Q. What are the withdrawal symptoms of Phentermine?

  1. Phentermine may cause mood swings, nausea, trembling, stomach upset, nervousness, and sleep problems.[xxv]

Q. Why do I have to take Phentermine on an empty stomach?

  1. An empty stomach helps the body absorb Phentermine better.

Q. Why I cannot take Phentermine with antidepressants?

  1. Taking Phentermine with antidepressants, notably monoamine oxidase inhibitor (MAOI) drugs, is associated with the development of problems in the heart valves. Do not take Phentermine if you have taken any antidepressant drug during the past 14 days.

Q. Can I give Phentermine to treat obesity in children?

  1. No. The safety and effectiveness of Phentermine to children is not studied. Phentermine can be harmful to children.

Q. Can I take water pills with Phentermine?

  1. Only do so when ordered by the doctor.

Q. Will Phentermine affect my libido?

  1. Some people taking Phentermine report reduced the desire for sex. If this bothers you, discuss it with your doctor.

Q. Can I use stool softeners to address constipation caused by Phentermine?

  1. Only do so when ordered by the doctor. If you have constipation, tell it to your doctor right away.

Q. Can I take Phentermine with Ritalin?

  1. Ritalin and Phentermine are both stimulants, and taking them together may trigger serious side effects. Do not take other drug or illicit substances while taking Phentermine. Only take drugs prescribed by your doctor.

[i] Phentermine.

[ii] Is phentermine a good option for weight loss? Mayo Clinic.

[iii] DEA Drug Fact Sheets. Drug Enforcement Agency.

[iv] Phentermine Dependence.

[v] Phentermine.

[vi] Ionamin.

[vii] Phentermine History.

[viii] “Fen-Phen” Update (Fenfluramine, Phentermine, Dexfenfluramine) FDA.

[ix] The History Of Phentermine. Dr. Alex Roig.

[x] Phentermine.

[xi] Adipex P.

[xii] Suprenza orally disintegrating tablets.

[xiii] Lomaira.

[xiv] Qysmia.

[xv] Phentermine Pregnancy and Breastfeeding Warnings.

[xvi] Phentermine.

[xvii] Phentermine Dosage.

[xviii] Phentermine Side Effects.

[xix] Phentermine Drug Interactions.

[xx] Phentermine Pregnancy Warnings.

[xxi] Phentermine Dosage.

[xxii] How To: Avoid Withdrawals with Phentermine. Phentermine.

[xxiii] What is Phentermine, and will it cause a positive result on a urine drug test? HPRC.

[xxiv] phentermine/topiramate Davis Drug Guide.

[xxv] What Are the Symptoms of Phentermine Withdrawal?. Wisegeek.