Levomethorphan is a drug very rarely used. It is the l-stereoisomer of methorphan. The effects of the two isomers are quite different. Dextromethorphan is an antitussive at low doses and a dissociative at much higher doses, whereas levomethorphan is an opioid analgesic. Levomethorphan has effects similar to levorphanol but is less potent as it must be demethylated to the active form by liver enzymes before becoming active.

Dextromethorphan is a common cough suppressant used throughout the world. But, in September 2013, 11 children in Paraguay were admitted to the hospital shortly after receiving cough syrup for their cold symptoms. These children experienced severe breathing problems, hallucinations and seizures - and over the following weeks, dozens of other Paraguayan children were hospitalized with the same alarming drug reactions. As it transpired, the culprit was not dextromethorphan, but contamination by the drug's mirror version. Levomethorphan has the same size, weight and chemical makeup as dextromethorphan, but with a reversed structure. This subtle change has major ramifications, as the compound is an opioid five times stronger than morphine. Paraguayan authorities and the World Health Organization (WHO) subsequently traced the defective drug back to a factory in India, which had previously shipped a similarly contaminated batch of cough syrup to Pakistan. "It was ultimately implicated in the deaths of more than 50 people in Pakistan," says Leonel Santos, director of chemical medicines at USP, also known as the United States Pharmacopeia. Fortunately, close vigilance by manufacturers and regulators mitigates and intercepts most drug impurities before they can harm people.

an opioid pain reliever that has never been marketed. It is about five times stronger than morphine.