Phentermine Side Effects

Medically reviewed by Drugs.com. Last updated on Nov 11, 2024.

Applies to phentermine: oral capsule, oral tablet.

Serious side effects of phentermine

Along with its needed effects, phentermine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor immediately if any of the following side effects occur while taking phentermine:

Rare side effects

• seeing, hearing, or feeling things that are not there
• severe mental changes

Incidence not known

• chest pain
• decreased ability to exercise
• dizziness
• fainting
• fast, irregular, pounding, or racing heartbeat or pulse
• headache
• numbness or tingling in the arms or legs
• swelling of the feet or lower legs
• trembling or shaking of the legs, arms, hands, or feet
• trouble breathing
• trouble with thinking, speaking, or walking
• weakness

Other side effects of phentermine

Some side effects of phentermine may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects.

Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:

Incidence not known

• decreased interest in sexual intercourse
• difficulty having a bowel movement (stool)
• dry mouth
• false or unusual sense of well-being
• hives or welts, itching, or skin rash
• inability to have or keep an erection
• increased in sexual ability, desire, drive, or performance
• increased interest in sexual intercourse
• loss in sexual ability, desire, drive, or performance
• redness of the skin
• unpleasant taste

For healthcare professionals

Applies to phentermine: compounding powder, oral capsule, oral capsule extended release, oral tablet, oral tablet disintegrating.

General adverse events

The more commonly reported adverse reactions have included palpitations, restlessness, insomnia, and headache.

Cardiovascular

• Frequency not reported: Regurgitant cardiac valvular disease, primary pulmonary hypertension, palpitations, tachycardia, elevated blood pressure, precordial pain, ischemic events, angina, myocardial infarction, cardiac failure, cardiac arrest^[Ref]

Nervous system

• Frequency not reported: Overstimulation, restlessness, nervousness, dizziness, euphoria, dysphoria, tremor, headache, fatigue, cerebrovascular events^[Ref]

Psychiatric

• Frequency not reported: Insomnia, psychosis, impotence, libido changes, depression, hallucinations^[Ref]

Hypersensitivity

• Frequency not reported: Urticaria^[Ref]

Gastrointestinal

• Frequency not reported: Dry mouth, unpleasant taste, diarrhea, constipation, other gastrointestinal disturbances, nausea, vomiting, abdominal cramps^[Ref]

Dermatologic

• Frequency not reported: Rash^[Ref]

Genitourinary

• Frequency not reported: Micturition disturbances^[Ref]

Other

• Frequency not reported: Facial edema^[Ref]

Frequently asked questions

• Why is phentermine a controlled substance?

Further information

Phentermine side effects can vary depending on the individual. Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Some side effects may not be reported. You may report them to the FDA.

Medical Disclaimer