Phentermine and topiramate
Anti-obesity drugs
Anti-obesity drugsPregnancy: contraindicated
Contraindicated: topiramate raises the risk of oral clefts; a negative pregnancy test is recommended before starting and monthly during treatment, with effective contraception.
An extended-release combination of an appetite-suppressing stimulant and an antiseizure drug, used long term with diet and exercise in obesity.
Quick facts
- Group
- Anti-obesity drugs
- Target
- Brain appetite and satiety circuits (phentermine releases catecholamines; topiramate acts on several targets)
- Used for
- Chronic weight management with diet
- Key effects
- Tingling of hands and feet · Altered taste · Insomnia
- Monitor
- Weight · Heart rate and blood pressure
- ATC
- A08AA51
No doses: the clinician sets them by the current guideline.
Mechanism of action
Phentermine is an amphetamine-like sympathomimetic that reduces appetite, likely through catecholamine release in the hypothalamus. Topiramate, an antiseizure drug, suppresses appetite and enhances satiety by mechanisms not fully known: it boosts GABA activity, modulates voltage-gated ion channels, blocks AMPA/kainate glutamate receptors and inhibits carbonic anhydrase.
- Phentermine releases catecholamines in appetite centres
- Topiramate adds satiety through several central actions
- Less food intake; weight falls with diet and exercise
- Topiramate also inhibits carbonic anhydrase (acidosis, stones)
Indications
- Chronic weight management with diet and exercise in obesity, or overweight with weight-related conditions Obesity →
Adverse effects
Common
- Tingling of hands and feet (paraesthesia)
- Altered taste, dry mouth
- Insomnia, dizziness
- Constipation
- Raised resting heart rate
Contraindications
- Pregnancy
- Glaucoma
- Hyperthyroidism
- During or within 14 days after an MAO inhibitor
- Hypersensitivity to phentermine, topiramate or sympathomimetic amines
Key interactions
- MAO inhibitors
- Risk of hypertensive crisis; contraindicated.
- Other carbonic anhydrase inhibitors (acetazolamide, zonisamide)
- More metabolic acidosis and kidney stones; avoided.
- Oral contraceptives
- Irregular bleeding (spotting) may occur; contraception is not stopped.
- Alcohol and CNS depressants
- More dizziness, drowsiness and cognitive effects.
What is monitored
- Weight, to judge benefit
- Heart rate and blood pressure
- Mood and suicidal thoughts
- Electrolytes and bicarbonate (acidosis)
- Monthly pregnancy test in people who could become pregnant
- Eye pain or sudden blurred vision
Administration and safety
- Oral extended-release capsule, once a day in the morning to limit insomnia.
- Started low and increased stepwise; higher amounts are stopped gradually, not abruptly, because of seizure risk.
- Controlled substance because of the phentermine component.
- Distributed with a risk programme because of fetal risk.
Pregnancy and breastfeeding
Contraindicated: topiramate raises the risk of oral clefts; a negative pregnancy test is recommended before starting and monthly during treatment, with effective contraception.
Nursing care
- Before: pregnancy test, glaucoma, thyroid, MAOI use, history of depression or kidney stones.
- During: heart rate, mood, eye symptoms, bicarbonate; monthly pregnancy test.
- Teach to drink plenty of fluids to lower kidney stone risk.
- Teach to report sudden eye pain or blurred vision, low mood or suicidal thoughts.
- Teach that it is tapered, not stopped suddenly.
Pharmacokinetics
- Absorption
- Oral, extended-release capsule taken in the morning.
- Elimination
- Both components are cleared largely by the kidneys; exposure rises in kidney impairment.
Test yourself
Think of the answer, then tap the question to see it.
Which component of phentermine-topiramate causes metabolic acidosis and kidney stones, and why?
Topiramate, because it inhibits carbonic anhydrase.
Why is the combination tapered rather than stopped suddenly?
Abrupt withdrawal of topiramate can trigger seizures, even in people without epilepsy.
What birth defect is linked to topiramate in pregnancy?
Oral clefts (cleft lip or palate).
What eye emergency can topiramate cause?
Acute myopia with secondary angle-closure glaucoma.
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 and Topiramate: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); DailyMed - PHENTERMINE AND TOPIRAMATE capsule, extended release (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.