A key committee is urging European regulators to turn down a sickle cell treatment—two years after their American counterparts approved the drug.

The EMA’s Committee for Medicinal Products for Human Use says its worried that the main trial that Emmaus Medical relied on to support its experimental drug Xyndari (glutamine) didn’t prove that the medicine was effective at reducing the number of sickle cell crises or hospital visits.

“A large number of patients, more who were taking Xyndari than taking placebo, dropped out of the study before it was finished, and information on how the medicine worked for those patients was not available,” the committee wrote in its recommendation Wednesday.

Committee members were also worried that the study didn’t have a large enough patient population and had concerns that many patients in the Xyndari group were also taking another sickle cell disease medicine, hydroxyurea, which may have influenced the results.