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Phentermine: 7 things you should know

Medically reviewed by Carmen Fookes, BPharm. Last updated on Dec 30, 2022.

1. How it works

  • Phentermine is a central nervous stimulant that may be used to treat obesity.
  • Experts aren't exactly sure how phentermine works but it appears to have multiple actions including stimulating neurons to release the neurotransmitters dopamine and norepinephrine, which may account for its appetite-suppressing effects. Phentermine also appears to increase levels of norepinephrine, dopamine, and serotonin by inhibiting their reuptake and inhibiting monoamine oxidase enzymes. It possibly also indirectly raises leptin levels in the brain (leptin is a hormone that tells us we feel full), and reduces the effects of neuropeptide Y - a chemical messenger that initiates eating, decreases energy expenditure, and increases fat storage.
  • Phentermine belongs to the class of drugs known as anorectics or anorexigenics.

2. Upsides

  • May be used short-term as an aid to weight loss in addition to exercise, behavioral modification, and caloric restriction.
  • Recommended only for people with an initial BMI equal to, or more than, 30 kg/m2, or 27 kg/m2 with risk factors for a cardiovascular event (such as controlled high blood pressure, diabetes, or high cholesterol).

3. Downsides

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

  • High blood pressure and other adverse cardiovascular effects. Restlessness, insomnia, headache, psychosis, dry mouth, rash, sexual dysfunction, an unpleasant taste, and gastrointestinal disturbances may also occur.
  • Treatment of more than a few weeks increases the risk of addiction and dependence on phentermine. Abrupt cessation following prolonged dosing may cause a withdrawal effect (symptoms include extreme fatigue and depression).
  • May impair judgment and affect a person's ability to drive or operate machinery. Alcohol should be avoided.
  • Tolerance may develop to the weight-loss effects of phentermine, rendering it less effective. The dosage of phentermine should not be increased in this situation, rather the drug may need to be discontinued under medical advice.
  • Should not be used in children aged 16 or less.
  • The dosage of phentermine requires reduction in people with severe kidney impairment and it should be avoided in people with end-stage kidney disease. Phentermine is not suitable for people with a history of cardiovascular or valvular heart disease, glaucoma, hyperthyroidism, a history of drug abuse, who are pregnant or breastfeeding, or with agitation.
  • May interact with several other drugs including other medications used for weight loss (including other prescribed medications, over-the-counter preparations, or herbal supplements), antidepressants, antipsychotics, antidiabetic drugs, and other central nervous system stimulants. Some interactions may be potentially fatal, such as primary pulmonary hypertension (PPH), a frequently fatal lung disease that can occur if phentermine is taken with fenfluramine or dexfenfluramine.

Note: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. View complete list of side effects

4. Bottom Line

Phentermine may be used short-term as an aid to weight loss; however, it can be addictive and tolerance may develop to its weight-losing effects, rendering it less effective with time.

5. Tips

  • Administer before breakfast or one-to-two hours after breakfast. Avoid late evening administration as this may cause insomnia. Some people may benefit from a twice-daily dosage regimen.
  • Your doctor will decide the appropriate dosage for you. Take phentermine exactly as directed and for the duration intended. If you find yourself gaining weight while taking phentermine see your doctor.
  • Phentermine should be used alongside dietary changes, exercise, and behavioral modification as discussed with your doctor.
  • Do not drive or operate machinery if this medicine impairs your judgment.
  • Do not drink alcohol while taking phentermine as it may increase the risk of side effects and further impair your judgment.
  • Should not be used at the same time as other medications used for weight loss.
  • Recommended only for people with an initial BMI of equal to, or more than, 30 kg/m2, or 27 kg/m2 with risk factors for a cardiovascular event (such as controlled high blood pressure, diabetes, or high cholesterol). Do not share your phentermine with anybody else (this is also illegal).
  • Report any new shortness of breath, angina pain, fluid retention in your legs or feet, or a decrease in your ability to exercise (not attributable to any other cause) to your doctor immediately.
  • Phentermine may be sought out by drug users and one accidental dose may be fatal to children or pets. Keep supplies out of reach and sight of others.
  • Tell your doctor if you are pregnant, intending to become pregnant, or breastfeeding because phentermine may not be suitable for you.

6. Response and effectiveness

  • Peak concentrations of phentermine are reached within three to four hours.
  • Studies have shown that, on average, phentermine helps people lose approximately one extra pound per week.
  • The rate of weight loss is greatest in the first few weeks of therapy.

7. Interactions

Medicines that interact with phentermine may either decrease its effect, affect how long it works, increase side effects, or have less of an effect when taken with phentermine. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.

Common medications that may interact with phentermine include:

  • amphetamine
  • antacids
  • antihypertensives, such as beta-blockers
  • atomoxetine
  • bupropion
  • carbonic anhydrase inhibitors
  • dextroamphetamine
  • dopamine
  • epinephrine
  • ethosuximide
  • isoproterenol
  • linezolid
  • lisdexamfetamine
  • lithium
  • methylphenidate
  • monoamine oxidase inhibitors
  • opioids
  • phenylephrine
  • phenytoin
  • selegiline
  • tranylcypromine
  • venlafaxine.

Avoid drinking alcohol or taking illegal or recreational drugs while taking phentermine.

Note that this list is not all-inclusive and includes only common medications that may interact with phentermine. You should refer to the prescribing information for phentermine for a complete list of interactions.


Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use phentermine only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Copyright 1996-2023 Revision date: December 30, 2022.