Drug-induced muscle spasm (DIMS) includes several somewhat overlapping concepts of true spasm, cramps, and dystonia. Spasms are involuntary muscle contractions. When prolonged and painful, they are often termed cramps. Acute dystonia is the most serious of the three concepts and may involve the limbs, trunk, face, or neck, resulting in often-painful postures. When acute dystonic reactions affect the neck region, difficulty swallowing or speaking can ensue. DIMSs often occur at drug initiation or with a rapid increase in dose.

Many medications have been associated with muscle spasms and cramps (see Drug Reaction Data below).

• Acute dystonic reactions are most commonly caused by neuroleptics, especially high-potency neuroleptics, but also antiemetics. The pathophysiology is most often linked with blockage of dopamine receptors.
• Diuretics may cause muscle spasm through dehydration or an electrolyte imbalance, especially hypokalemia, hypocalcemia, or hypomagnesemia.
• Muscle spasm can accompany myopathy, which has been associated with numerous medication classes including antimalarials and lipid-lowering agents.

Dehydration, muscle fatigue, magnesium or potassium deficiency, or an underlying neuropathy may predispose to worsening DIMS. Other risk factors for acute dystonia include young age and male sex.