Adverse effects from counterfeit and mislabeled medicine containing tapentadol and carisoprodol.

When self-administration with counterfeit or mislabeled medicine is suspected, comprehensive laboratory analysis should be preferred over immunoassay screening to avoid false negative results. Carisoprodol, which was formerly a popular muscle relaxant drug in many countries, has reappeared on illegal drug markets, and may cause an itching, purple-colored rash, even after a single dose.