A hung jury forced a mistrial in Los Angeles on Thursday in the cases of two men who blamed their heart attacks on Vioxx, the once-popular painkiller made by pharmaceutical company Merck & Co. that is now the subject of thousands of lawsuits.

It wasn’t clear when there might be a retrial of the lawsuits brought by Lawrence Appell, of Scottsdale, Ariz., and Rudolph Arrigale, of Westminster, Calif. They argued that Vioxx was a substantial factor in their heart problems and that Merck failed to give sufficient warning of potential safety hazards of the drug.