[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Pharmacology](https://durgical.com/en/pharmacology/) › [Anti-obesity drugs](https://durgical.com/en/pharmacology/classes/anti-obesity-drugs/) › Phentermine # Phentermine ## Anti-obesity drugs? Anti-obesity drugs ## Pregnancy: 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. Updated Nov 1, 2026 Also called: phentermine hydrochloride ## Quick facts **Group** [Anti-obesity drugs](https://durgical.com/en/pharmacology/classes/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. On this page - The basics Mechanism of action - Indications - Adverse effects - Pharmacokinetics - Memory aid - Clinical What the labels mean - Safety Contraindications - Key interactions - What is monitored - Administration and safety - Pregnancy and breastfeeding - Nursing care - Study Test yourself - Connections Connections ## 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 →](https://durgical.com/en/conditions/obesity/) ## Adverse effects ### Common - Dry mouth and unpleasant taste - Insomnia, restlessness, tremor - Raised heart rate and blood pressure, palpitations - Constipation or diarrhoea ### Serious (rare, important) - Primary pulmonary hypertension (rare) - Valvular heart disease (rare; mainly reported with fenfluramine combinations) - Abuse and dependence - Ischaemic events ## 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. ## Memory aid PHENtermine = 'PHEN' of the old 'fen-phen': an amphetamine cousin that cuts appetite for a few weeks, then tolerance wins. Heart, thyroid, glaucoma, MAOIs and pregnancy say no. ## 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. [Add to my cards (3)](https://durgical.app/estudiar/?add=/en/pharmacology/phentermine/) [More Pharmacology questions →](https://durgical.com/en/practice/?f=pharmacology) ## What the labels mean **Pregnancy: contraindicated** Contraindicated: weight loss offers no benefit in pregnancy and may harm the fetus. ## Connections ## Related drugs 1 - [Orlistat](https://durgical.com/en/pharmacology/orlistat/) ## Related conditions 1 - [Obesity](https://durgical.com/en/conditions/obesity/) [← Back to the class: Anti-obesity drugs](https://durgical.com/en/pharmacology/classes/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](https://medlineplus.gov/druginfo/meds/a682187.html) (MedlinePlus (U.S. National Library of Medicine), 2024); [DailyMed - PHENTERMINE HYDROCHLORIDE capsule](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=e4508aa9-1e7a-4af7-829e-5fa353c92f5e) (DailyMed (U.S. National Library of Medicine), 2026); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=A08AA01) (WHO Collaborating Centre for Drug Statistics Methodology, 2026). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/pharmacology/phentermine/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/phentermine/