What are the most common side effects of Phentermine?

The label names raised heart rate, raised blood pressure, restlessness, dizziness, insomnia, headache and others, by organ system and in prose. It reports no incidence rates for any of them, so there is no frequency table to link and no threshold behind this list.

Who should not take it?

The label contraindicates it in these cases: A history of cardiovascular disease — coronary artery disease, stroke, arrhythmia, heart failure or uncontrolled hypertension; Use with a monoamine oxidase inhibitor, or within 14 days of stopping one; Hyperthyroidism; Glaucoma; Agitated states; A history of drug abuse; Pregnancy and breastfeeding; Known hypersensitivity or idiosyncrasy to the sympathomimetic amines.

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Phentermine (generic) side effects

Phentermine's FDA label lists 9 reactions, 5 serious warnings and 8 contraindications. All of them are below in the label's own order; any percentage is the label's own figure, and nothing here is a recommendation.

No clinician has reviewed this page. Label and trial statements are quoted from the FDA label or the trial cited beside them. General guidance is marked as such, and every price carries the date it was verified. That is sourcing, not medical review. What we do and do not check · Who checked the price

Last updated 2026-06-15 — the version of the FDA label this page quotes. Read and dated by Alex Seot.

What are the side effects of Phentermine?

The FDA label for Phentermine names raised heart rate, raised blood pressure, restlessness, dizziness and insomnia among its adverse reactions, by organ system and without incidence rates: the label carries no percentages for any of them. The label carries no boxed warning, which is a fact about this label and not a statement that the drug is without risk. The label's serious warnings include primary pulmonary hypertension; valvular heart disease; withdrawal effects after prolonged high dosage — extreme fatigue and depression; and impaired ability to drive or operate machinery. It is contraindicated in 8 cases, among them a history of cardiovascular disease — coronary artery disease, stroke, arrhythmia, heart failure or uncontrolled hypertension, meaning the label says not to prescribe it at all. Every line above is quoted from FDA prescribing information for phentermine hydrochloride tablets (Aurolife Pharma) rather than summarized from a secondary source, because a side-effect list paraphrased twice stops being the label.

Common, and usually manageable

Reactions the label names. It gives no incidence rates, so this is a list without a threshold.

raised heart rate
raised blood pressure
restlessness
dizziness
insomnia
headache
dry mouth
diarrhea
constipation

The label approves it as a short-term adjunct, a few weeks. Every monthly subscription selling it as ongoing therapy is using it beyond that duration, and the withdrawal the label describes is what makes leaving one harder than joining it.

Serious, and worth knowing before you start

Primary pulmonary hypertension
Valvular heart disease
Withdrawal effects after prolonged high dosage — extreme fatigue and depression
Impaired ability to drive or operate machinery
Dependence: it is a Schedule IV controlled substance related to the amphetamines

Who cannot take it

A history of cardiovascular disease — coronary artery disease, stroke, arrhythmia, heart failure or uncontrolled hypertension
Use with a monoamine oxidase inhibitor, or within 14 days of stopping one
Hyperthyroidism
Glaucoma
Agitated states
A history of drug abuse
Pregnancy and breastfeeding
Known hypersensitivity or idiosyncrasy to the sympathomimetic amines

Each of these, on its own

What the label says, why it happens, how it tracks the dose schedule and the point at which it stops being ordinary.

What it costs Dosage schedule

Questions

What are the most common side effects of Phentermine?: The label names raised heart rate, raised blood pressure, restlessness, dizziness, insomnia, headache and others, by organ system and in prose. It reports no incidence rates for any of them, so there is no frequency table to link and no threshold behind this list.
Who should not take it?: The label contraindicates it in these cases: A history of cardiovascular disease — coronary artery disease, stroke, arrhythmia, heart failure or uncontrolled hypertension; Use with a monoamine oxidase inhibitor, or within 14 days of stopping one; Hyperthyroidism; Glaucoma; Agitated states; A history of drug abuse; Pregnancy and breastfeeding; Known hypersensitivity or idiosyncrasy to the sympathomimetic amines.

Source: FDA prescribing information for phentermine hydrochloride tablets (Aurolife Pharma), the version published 2026-06-15. Information only, not medical advice: a side effect you are having is a conversation with your prescriber, not with a comparison site.