Antibiotics and Oral Contraceptive — MFDS Part 1 MCQ

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ModeratePharmacologyAntibiotics and Oral ContraceptiveMFDS Part 1

Which of the following antibacterial drugs is most likely to reduce the contraceptive efficacy of the combined oral contraceptive pill?

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Correct answer: A — Rifampicin

Rifampicin is the correct answer. It is a potent inducer of hepatic drug-metabolising enzymes and transporters, increasing the metabolism of oestrogens and progestogens and thereby reducing systemic exposure to combined oral contraceptives. Amoxicillin, metronidazole, erythromycin and azithromycin are not enzyme-inducing antibacterials and do not routinely reduce contraceptive efficacy through this mechanism. Additional precautions are therefore not required solely because one of these non-enzyme-inducing antibiotics has been prescribed. However, vomiting or severe diarrhoea caused by either the illness or treatment can impair absorption of an oral contraceptive; in that situation, the relevant missed-pill or gastrointestinal-disturbance advice should be followed.

Reference: Electronic Medicines Compendium, Rifampicin 300 mg Capsules, Summary of Product Characteristics, section 4.5: Interaction with other medicinal products, updated 22 May 2026. https://www.medicines.org.uk/emc/product/8789/smpc