Drugs contraindicated in myasthenia gravis

1. Big picture

In myasthenia gravis (MG), neuromuscular transmission is already weak because antibodies impair the postsynaptic neuromuscular junction, most commonly the acetylcholine receptor.

Therefore, some drugs can worsen MG by:

• reducing acetylcholine release;
• blocking neuromuscular transmission;
• worsening muscle weakness;
• increasing respiratory depression;
• increasing secretions;
• triggering myasthenic crisis.

The key exam sentence:

In myasthenia gravis, avoid drugs that impair neuromuscular transmission, especially aminoglycosides, fluoroquinolones, macrolides, magnesium, beta-blockers, antiarrhythmics, neuromuscular blockers, botulinum toxin, penicillamine, lithium, sedatives and some antiepileptics.

Important practical point:

Many “contraindicated” drugs are relative contraindications. If a drug is life-saving, it may be used with specialist supervision and respiratory monitoring.

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