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What Are the Ingredients in the Phentermine Diet Drug?

author image Barb Nefer
Based in Kissimmee, Fla., Barb Nefer is a freelance writer with over 20 years of experience. She is a mental health counselor, finance coach and travel agency owner. Her work has appeared in such magazines as "The Writer" and "Grit" and she authored the book, "So You Want to Be a Counselor."
What Are the Ingredients in the Phentermine Diet Drug?
Phentermine is a weight loss aid. Photo Credit Stockbyte/Stockbyte/Getty Images

Eating moderate amounts of nutritious food and getting regular exercise are the healthiest ways to lose weight, but some people turn to diet drugs to help them drop more pounds. Some diet pills are available over the counter. Phentermine is a medication that requires a prescription and is used under a doctor's supervision as part of an overall weight loss plan.


The active ingredient in the phentermine diet pill is a stimulant drug that is very similar in composition to amphetamine. It belongs to a drug class called anorectics, according to PubMed Health. Drugs in this category work on your central nervous system and cut down your appetite, allowing you to lose weight because you eat less. Phentermine can be habit forming, so it should only be used as prescribed and should never be shared with another person. Typically the drug is only used for three to six weeks.

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The active ingredient in phentermine reduces appetite, so this drug is prescribed to obese people who have other health-related risk factors like diabetes, high blood pressure and elevated cholesterol levels, Drugs.com reports. Phentermine is not used in people with glaucoma, an overactive thyroid, heart disease or coronary artery disease, and those with diabetes or high blood pressure must be carefully monitored. Some people are allergic to the active ingredient in this drug.

Side Effects

Phentermine diet pills cause unpleasant side effects in some users. Typical problems include blurry vision, dry mouth, dizziness, problems sleeping, anxiety and constipation, according to Mayo Clinic physician Dr. Donald Hensrud. Your doctor may be able to lessen or eliminate the side effects by adjusting your dose.


While the active ingredient in phentermine cuts down your desire to eat, it is not meant as a long-term weight loss solution and should not be relied on as your sole means of losing weight. Your doctor should provide instructions for an exercise program and eating plan when you get your prescription. Follow this plan carefully and continue it after you stop taking phentermine, to maintain your weight loss.


The active ingredient in phentermine is safe for most people if used correctly, but it can trigger serious side effects when combined with the drugs fenfluramine or dexfenfluramine. These drugs were once commonly prescribed together, but they caused a fatal lung disorder in some patients, Drugs.com warns. Phentermine may also interact with other common drugs like MAO inhibitors, so you must tell your doctor about any other medications you are taking if you get a prescription for the diet drug.

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