Atypical myopathy: pentazocine induced.

Pentazocine is a mixed agonist/antagonist opioid analgesic agent that can induce an atypical myopathy following intramuscular injections. This iatrogenic myopathy is rare, typified by fibrosis, and should be differentiated from other myopathies. We describe a case of pentazocine-induced myopathy that caused wasting of musculature in the gluteal region bilaterally and stiffness and woody induration in the legs of 42-year-old man. Though rare, clinicians should be alerted to this potential side effect and avoid repeated administration of intramuscular pentazocine.