6 known interactions • 2 major • 4 moderate • 0 minor
Always disclose all medications to your healthcare providers — prescription medicines, OTC medications, vitamins, and herbal supplements. This list may not include every possible interaction. Use our Medicine Interaction Checker to screen your complete medication list.
Potentially life-threatening or causing permanent damage. Avoid combination.
May worsen condition or require dose adjustment. Monitor closely.
Usually limited clinical effect. Manage with routine monitoring.
MAO inhibitors (phenelzine, selegiline, rasagiline)
Concurrent or recent MAOI use with phentermine risks severe hypertensive crisis and hyperthermia.
Management: Contraindicated. Do not use within 14 days of stopping an MAOI.
Other sympathomimetics / stimulants (pseudoephedrine, amphetamines, caffeine excess)
Additive cardiovascular and CNS stimulation; marked increase in hypertension and arrhythmia risk.
Management: Avoid combination. Counsel patients to limit caffeine.
SSRIs / SNRIs / tricyclic antidepressants
Serotonergic agents combined with phentermine raise serotonin syndrome risk; tricyclics may worsen arrhythmia.
Management: Use cautiously; monitor for serotonin syndrome signs (agitation, tachycardia, diaphoresis).
Insulin / oral antidiabetics
Weight loss from phentermine may reduce insulin requirements; untreated leads to hypoglycemia.
Management: Monitor blood glucose closely; reduce antidiabetic doses as weight is lost.
Antihypertensives (guanethidine, methyldopa, clonidine)
Phentermine may antagonize antihypertensive effects through adrenergic stimulation.
Management: Monitor blood pressure; adjust antihypertensive doses as needed.
Alcohol
Combination may unpredictably alter CNS effects of phentermine.
Management: Advise patients to avoid alcohol while on phentermine.
Always ask your pharmacist about potential interactions with food, alcohol, and supplements specific to Phentermine. Some medicines have significant interactions with grapefruit juice, high-fat meals, dairy products, or vitamin K-rich foods.