An evaluation of the US Food and Drug Administration's Adverse Event Reporting System identified that patients coinfected with human immunodeficiency virus and chronic hepatitis C virus who were treated with a regimen of ribavirin and didanosine, with or without stavudine, were at increased risk for events associated with mitochondrial toxicity, including fatal hepatic failure, peripheral neuropathy, pancreatitis, and symptomatic hyperlactatemia/lactic acidosis. In response, the US product labels for didanosine and ribavirin have been revised to caution clinicians against coadministration of these drugs.