FILE PHOTO: Logo of Swiss drugmaker Novartis is seen at its branch in Schweizerhalle near Basel, Switzerland, March 29, 2018. REUTERS/Arnd Wiegmann

(Reuters) - Novartis AG's NOVN.S Sandoz unit said on Wednesday it was halting distribution of its versions of the drug commonly known as Zantac in all its markets, including the United States and Canada, after contaminants were found in the heartburn drug.

The Swiss drugmaker’s steps follow an investigation by U.S. and European regulators into the presence of the impurity, N-nitrosodimethylamine (NDMA), in the drug, ranitidine, and a distribution halt in Canada announced late Tuesday.

Health Canada said it had requested makers of the drug to stop distribution as it gathers more information and consults with international health regulators.

Branded versions of the drug such as Sanofi SA's SASY.PA Zantac are available for over-the-counter sale in Canada, while generic versions are available for sale both OTC and with prescription.

A distribution halt is different from a recall and means that existing stock of the drugs in pharmacies or stores can still be sold.

Neither the U.S. Food and Drug Administration nor the European Medicines Agency had asked patients to stop taking the drug when the regulators said last week they were reviewing the safety of ranitidine.

Novartis said in an emailed statement its step was precautionary and adequate measures “will be implemented in alignment with relevant health authorities as required.”