Phentermine
Anti-obesity drugs
Anti-obesity drugsPregnancy: contraindicated
Contraindicated: weight loss offers no benefit in pregnancy and may harm the fetus.
An amphetamine-like appetite suppressant used for a few weeks alongside diet and exercise in obesity. It is a controlled drug.
Also called: phentermine hydrochloride
Quick facts
- Group
- Anti-obesity drugs
- Target
- Noradrenergic nerve endings in the brain (releases noradrenaline)
- Used for
- Short-term
- Key effects
- Dry mouth and unpleasant taste · Insomnia
- Monitor
- Weight · Blood pressure and heart rate
- ATC
- A08AA01
No doses: the clinician sets them by the current guideline.
Mechanism of action
Phentermine is a sympathomimetic amine related to amphetamine. It is thought to reduce appetite mainly by increasing noradrenaline release in hypothalamic appetite centres; other central and metabolic effects may contribute. Like other drugs of its family it stimulates the CNS and raises blood pressure, and tolerance to the appetite effect develops within weeks.
- Enters noradrenergic nerve endings like an amphetamine
- Releases noradrenaline in the hypothalamus
- Appetite falls; sympathetic tone rises
- Tolerance to the appetite effect appears within weeks
Indications
- Short-term (a few weeks) add-on to diet, exercise and behaviour change in obesity, or overweight with risk factors Obesity →
Adverse effects
Common
- Dry mouth and unpleasant taste
- Insomnia, restlessness, tremor
- Raised heart rate and blood pressure, palpitations
- Constipation or diarrhoea
Contraindications
- Cardiovascular disease (coronary disease, stroke, arrhythmias, heart failure, uncontrolled hypertension)
- During or within 14 days after an MAO inhibitor
- Hyperthyroidism
- Glaucoma
- Agitated states or a history of drug abuse
- Pregnancy and breastfeeding
Key interactions
- MAO inhibitors
- Risk of hypertensive crisis; contraindicated during and for two weeks after an MAOI.
- Insulin and oral antidiabetic drugs
- Requirements may fall as weight drops; hypoglycaemia is watched for.
- Adrenergic neuron blockers (guanethidine type)
- Their blood-pressure-lowering effect may be reduced.
- Alcohol
- May cause an adverse reaction.
What is monitored
- Weight, to judge whether to continue
- Blood pressure and heart rate
- New breathlessness, chest pain, fainting or leg swelling (pulmonary hypertension, valve disease)
- Signs of misuse
Administration and safety
- Oral, usually in the morning, to limit insomnia.
- Extended-release forms are not split, chewed or crushed.
- Controlled substance (Schedule IV in the US); only small quantities are dispensed.
- Not combined with other weight-loss drugs except approved combinations.
Pregnancy and breastfeeding
Contraindicated: weight loss offers no benefit in pregnancy and may harm the fetus.
Nursing care
- Before: blood pressure, heart history, thyroid disease, glaucoma, MAOI use and pregnancy status.
- During: blood pressure, pulse and sleep; report exercise intolerance or breathlessness.
- Teach that the appetite effect fades within weeks and the dose is not increased to chase it.
- Teach that it may impair driving and that alcohol is avoided.
Pharmacokinetics
- Absorption
- Oral; peak levels a few hours after a dose.
- Elimination
- Mostly in urine; exposure rises in kidney impairment.
Test yourself
Think of the answer, then tap the question to see it.
Why is phentermine approved only for a few weeks?
Tolerance to its appetite effect develops within weeks, and long-term safety is not established for the single drug.
What are three contraindications to phentermine?
Cardiovascular disease, use of an MAO inhibitor in the past 14 days, hyperthyroidism (also glaucoma, agitation, drug abuse history, pregnancy).
A patient on phentermine reports new breathlessness on exertion. What must be considered?
Primary pulmonary hypertension or valvular heart disease; the drug is stopped and the patient evaluated.
Connections
← Back to the class: Anti-obesity drugs
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: Phentermine: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2024); DailyMed - PHENTERMINE HYDROCHLORIDE capsule (DailyMed (U.S. National Library of Medicine), 2026); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026).
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Representative model, for educational purposes only.