A rare side effect is cropping up in a small number of European patients taking Johnson & Johnson's ( JNJ) anemia-fighting blockbuster drug Eprex, known in the U.S. as Procrit. The lead article in the Feb. 14 edition of the New England Journal of Medicine documents 13 patients on Eprex who developed pure red cell aplasia -- a condition in which the body stops producing red blood cells. Simply put, these patients develop an antibody reaction to Eprex, so instead of curing their anemia, the drug actually makes it worse. An accompanying editorial in the influential medical journal says that at least another 29 Eprex patients in Europe also have developed the same side effect. The patients all were diagnosed between 1998 and 2000. At this point, the occurrence of the side effect is rare, but Eprex sales -- and J&J's stock -- could be hurt if further evidence linking Eprex to pure red cell aplasia is discovered. Shares of J&J closed Wednesday at $58.09, up 79 cents. J&J first made public the new side effect in November, when it sent warning letters to European doctors. At the time, the drug giant downplayed the problem because the rate of incidence is so low -- less than 1 patient in 10,000. But the New England Journal of Medicine coverage is bringing the issue back into the spotlight. Eprex and Procrit, combined, are J&J's bestselling drugs, racking up $3.4 billion in sales last year. More than 3 million patients worldwide have used the drugs. Carol Goodrich, a J&J spokeswoman, said there is nothing new in the Journal of Medicine article, but because doctors were alerted to the issue, the number of reported European cases of pure red cell aplasia has grown to just under 100. "No cause and effect between Eprex and pure red cell aplasia has been proven yet, but we're working with regulatory authorities to examine a number of different factors," she says.