Adverse effect of verapamil in myasthenia gravis.

Verapamil, a class IV anti-arrhythmic drug that blocks voltage-dependent calcium channels in cardiac and smooth muscle, also has effects on presynaptic and postsynaptic voltage-dependent calcium channels at the neuromuscular junction. In a postoperative patient with pre-existent myasthenia gravis, oral verapamil caused a marked exacerbation in myasthenic weakness.