PharmacologyTier 1Medical Sciences concept

Antitubercular drug adverse effects (ethambutol-optic neuropathy, isoniazid, rifampicin, pyrazinamide)

Core concept

Four first-line agents (RIPE: rifampicin, isoniazid, pyrazinamide, ethambutol), each with a distinct signature toxicity -- matching is the exam task
All four can cause hepatotoxicity to varying degrees -- not the discriminating feature alone
Baseline and monitoring: LFTs, visual acuity/colour vision (ethambutol), before and during therapy

Key detail

Drug	Signature adverse effect	Mechanism/note
Ethambutol	Optic neuropathy -- dec visual acuity, red-green colour blindness	Dose-related; reversible if caught early -- monthly visual acuity/colour vision checks
Isoniazid	Peripheral neuropathy	Competes with pyridoxine (B6) metabolism -- co-prescribe pyridoxine to prevent; also hepatotoxicity, drug-induced lupus
Rifampicin	Orange/red body secretions, potent CYP450 inducer	Induces metabolism of OCP, warfarin, antiretrovirals -> reduced efficacy; flu-like reaction
Pyrazinamide	Hepatotoxicity (most hepatotoxic of the four), hyperuricaemia/gout	Inhibits urate excretion

Clinical relevance

Visual symptoms on ethambutol -> stop immediately and refer -- delay risks permanent visual loss
Peripheral neuropathy on isoniazid -> ensure pyridoxine co-prescribed (routine in most regimens), especially in diabetes, alcohol excess, pregnancy, HIV, malnutrition
Rifampicin's enzyme induction is a common exam trap: reduces efficacy of hormonal contraception and warfarin -- needs dose adjustment or alternative contraception

Correlations

Hepatotoxicity ranking (highest to lowest): pyrazinamide > isoniazid > rifampicin -- relevant when deciding which drug to stop first if LFTs rise
Rifampicin CYP induction overlaps with other inducers (carbamazepine, phenytoin, St John's wort) -- same principle, different drugs
Isoniazid neuropathy prevention (pyridoxine) parallels other B6-related neuropathies (e.g. hydralazine)

3 more sections, plus exam facts

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