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Phentermine is a fairly old prescription diet drug that has made a recent comeback.

Although not an amphetamine, it is similar in many respects and is classified as a controlled substance.

Phentermine is approved for relatively short-term use as an appetite suppressant in a calorie-restricted weight loss program.

Phentermine became immensely popular as part of the combination fen-phen. Bur reports of heart valve problems and primary pulmonary hypertension led to the withdrawal of fenfluramine, the other part of that combination.

Side Effects and Interactions

Phentermine can cause reactions such as palpitations, rapid heart beat, higher blood pressure, overstimulation, insomnia, restlessness, dizziness, dry mouth, or diarrhea.

Other reactions that have been reported include headache, tremor, constipation, digestive disturbances, metallic taste, hives, impotence, and alteration in sex drive.

A few people on phentermine have suffered strokes. Report any symptoms or suspected side effects to the physician promptly.

Phentermine should not be taken with an MAO inhibitor such as Nardil or Parnate.

Phentermine may also interact with insulin, many diabetes pills, and the blood pressure pill guanethidine.

Phentermine may react with beta blockers such as propranolol.

Some sources advise against alcohol consumption for people on phentermine. Check with your doctor and pharmacist before taking any other medicine while taking phentermine.

Special Precautions

Phentermine is inappropriate for people with glaucoma, serious high blood pressure, symptomatic heart disease, or hyperthyroidism.

Phentermine must not be combined with MAO inhibitor drugs (Eldepryl, Nardil, Parnate) and a person should wait at least 14 days after stopping any of these before starting to take phentermine.

Phentermine is not recommended for people in agitated states of mind or who have problems with drug abuse or alcoholism.

The effectiveness of this drug against the sensation of hunger may wear off after a few weeks.

The dose should not be raised in an attempt to maintain the effect; abuse of the drug can result in skin problems, insomnia, irritability, hyperactivity, and personality changes or even psychosis.

Sudden withdrawal may result in depression and extreme fatigue.

Taking the Medicine

Phentermine is taken once a day, usually two hours after breakfast.

Ionamin (phentermine resin, which is absorbed more slowly) is to be taken before breakfast, or 14 hours before bedtime.

The capsules are to be swallowed whole.

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About the Author
Joe Graedon is a pharmacologist who has dedicated his career to making drug information understandable to consumers. His best-selling book, The People’s Pharmacy, was published in 1976 and led to a syndicated newspaper column, syndicated public radio show and web site. In 2006, Long Island University awarded him an honorary doctorate as “one of the country's leading drug experts for the consumer.” .
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