Medically reviewed on Jul 26, 2018
What is phentermine?
Phentermine is similar to an amphetamine. Phentermine stimulates the central nervous system (nerves and brain), which increases your heart rate and blood pressure and decreases your appetite.
Phentermine may also be used for purposes not listed in this medication guide.
You should not use this medicine if you have glaucoma, overactive thyroid, severe heart problems, uncontrolled high blood pressure, advanced coronary artery disease, extreme agitation, or a history of drug abuse.
Before taking this medicine
You should not use phentermine if you are allergic to it, or if you have:
a history of heart disease (coronary artery disease, heart rhythm problems, congestive heart failure, stroke);
severe or uncontrolled high blood pressure;
extreme agitation or nervousness;
a history of drug abuse; or
if you take other diet pills.
Do not use phentermine if you have used an MAO inhibitor in the past 14 days. A dangerous drug interaction could occur. MAO inhibitors include isocarboxazid, linezolid, methylene blue injection, phenelzine, rasagiline, selegiline, tranylcypromine, and others.
You should not breast-feed while using phentermine.
Tell your doctor if you have ever had:
heart disease or coronary artery disease;
a heart valve disorder;
high blood pressure;
diabetes (your diabetes medication dose may need to be adjusted); or
Phentermine is not approved for use by anyone younger than 16 years old.
How should I take phentermine?
Follow all directions on your prescription label and read all medication guides or instruction sheets. Your doctor may occasionally change your dose. Use the medicine exactly as directed.
Phentermine is usually taken before breakfast, or 1 to 2 hours after breakfast. Follow your doctor's dosing instructions very carefully.
Never use phentermine in larger amounts, or for longer than prescribed. Taking more of this medication will not make it more effective and can cause serious, life-threatening side effects.
Phentermine is for short-term use only. The effects of appetite suppression may wear off after a few weeks.
Phentermine may be habit-forming. Misuse can cause addiction, overdose, or death. Selling or giving away this medicine is against the law.
Call your doctor at once if you think this medicine is not working as well, or if you have not lost at least 4 pounds within 4 weeks.
Do not stop using phentermine suddenly, or you could have unpleasant withdrawal symptoms. Ask your doctor how to safely stop using this medicine.
Store at room temperature away from moisture and heat. Keep the bottle tightly closed when not in use.
What happens if I miss a dose?
Take the medicine as soon as you can, but skip the missed dose if it is late in the day. Do not take two doses at one time.
What happens if I overdose?
Seek emergency medical attention or call the Poison Help line at 1-800-222-1222. An overdose of phentermine can be fatal.
Overdose symptoms may include confusion, panic, hallucinations, extreme restlessness, nausea, vomiting, diarrhea, stomach cramps, feeling tired or depressed, irregular heartbeats, weak pulse, seizure, or slow breathing (breathing may stop).
What should I avoid while taking phentermine?
Avoid driving or hazardous activity until you know how this medicine will affect you. Your reactions could be impaired.
Drinking alcohol with this medicine can cause side effects.
Phentermine side effects
Get emergency medical help if you have signs of an allergic reaction: hives; difficult breathing; swelling of your face, lips, tongue, or throat.
Call your doctor at once if you have:
feeling short of breath, even with mild exertion;
chest pain, feeling like you might pass out;
swelling in your ankles or feet;
pounding heartbeats or fluttering in your chest;
tremors, feeling restless, trouble sleeping;
unusual changes in mood or behavior; or
Common side effects may include:
dry mouth, unpleasant taste;
diarrhea, constipation, stomach pain; or
increased or decreased interest in sex.
This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.
What other drugs will affect phentermine?
Taking phentermine together with other diet medications such as fenfluramine (Phen-Fen) or dexfenfluramine (Redux) can cause a rare fatal lung disorder called pulmonary hypertension. Do not take phentermine with any other diet medications without your doctor's advice.
Many drugs can affect phentermine. This includes prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible interactions are listed here. Tell your doctor about all your current medicines and any medicine you start or stop using.
Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use this medication only for the indication prescribed.
Copyright 1996-2018 Cerner Multum, Inc. Version: 10.01.
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