Four patients were hospitalized in Israel in recent months with damage to their bone marrow, a suspected side-effect of taking the painkiller Optalgin.

The drug, maufactured by Teva Pharmaceuticals, contains the active ingredient dipyrone, which is considered dangerous to white blood cells and the bone marrow. In many countries, Dipyrone-based medications are banned or restricted so that they are available only with a prescription. In Israel, however, the drug is available over the counter.

“This is a known complication of Optalgin,” Professor Michael Lishner, head of the Department of Medicine at the Meir Hospital in Kfar Saba, was quoted by Channel 2 News as saying on Thursday. “This medicine was prohibited in the United States many years ago. According to the literature, the mortality rate in patients who used Optalgin and suffered similar symptoms were up to 10%.”

A Health Ministry probe was underway, with findings that are reportedly to be made public by next week. One possible outcome could be a decision to ban the marketing of Optalgin in Israel, or make it a prescription-only drug.

A Teva spokesperson said in response: “We regret the cases and will cooperate fully with the health authorities. Details at this stage are unknown to us, but the data collected over the years indicates this side effect is extremely rare, an estimated one in many millions. The efficacy and safety of the product, which has been marketed worldwide for decades, have been tested and continue to be tested by the ministry.”