[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 and topiramate

# Phentermine and topiramate

## Anti-obesity drugs?

Anti-obesity drugs

## Pregnancy: 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.

Updated Nov 1, 2026

## Quick facts

**Group**
[Anti-obesity drugs](https://durgical.com/en/pharmacology/classes/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.

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 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 →](https://durgical.com/en/conditions/obesity/)

## Adverse effects

### Common

- Tingling of hands and feet (paraesthesia)
- Altered taste, dry mouth
- Insomnia, dizziness
- Constipation
- Raised resting heart rate

### Serious (rare, important)

- Embryo-fetal toxicity (oral clefts with topiramate)
- Suicidal thoughts, mood and sleep disorders
- Acute myopia and secondary angle-closure glaucoma
- Cognitive slowing (attention, memory, word-finding)
- Metabolic acidosis and kidney stones
- Seizures if stopped abruptly

## 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.

## Memory aid

A two-drug pair: PHEN cuts appetite, TOP adds satiety, but TOPiramate brings its own baggage: 'tingles, taste, thinking, acid, stones, clefts'.

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/pharmacology/phentermine-and-topiramate/)

[More Pharmacology questions →](https://durgical.com/en/practice/?f=pharmacology)

## What the labels mean

**Pregnancy: contraindicated**
Contraindicated: topiramate raises the risk of oral clefts; a negative pregnancy test is recommended before starting and monthly during treatment, with effective contraception.

## Connections

## Related drugs 4

- [Phentermine](https://durgical.com/en/pharmacology/phentermine/)
- [Topiramate](https://durgical.com/en/pharmacology/topiramate/)
- [Naltrexone and bupropion](https://durgical.com/en/pharmacology/naltrexone-and-bupropion/)
- [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 and Topiramate: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a612037.html) (MedlinePlus (U.S. National Library of Medicine), 2026); [DailyMed - PHENTERMINE AND TOPIRAMATE capsule, extended release](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=918df93d-e033-4419-adcc-e21655ef8158) (DailyMed (U.S. National Library of Medicine), 2026); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=A08AA51) (WHO Collaborating Centre for Drug Statistics Methodology, 2026).

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/phentermine-and-topiramate/