SINGAPORE - Three brands of a medicine prescribed for high blood pressure are being recalled because they contain higher than acceptable amounts of nitrosamine impurity, which can potentially cause cancer.

However, patients currently taking these three brands of losartan are advised not to stop taking the medicine, as the immediate health risk is low, the Health Sciences Authority said in a statement on Thursday (March 28).

The drugs affected are the 50mg and 100mg tablets from these brands: Hyperten, Losagen and Losartas.

The Ministry of Health (MOH) said that about 137,000 patients in Singapore are using the three recalled brands of losartan.

Of these, about 130,000 patients have been prescribed Losartas at public hospitals and polyclinics.

With supplies of the safe varieties of losartan expected to be affected by the recall, the MOH has advised doctors to prescribe the medication on a one-month basis so that all patients have enough medicine to control their high blood pressure.

It said: "This ensures that patients in all settings are able to promptly receive the medication they need. This may be the practice for the next six months."

It added that patients whose next appointment is before July 1 should wait until then to speak to their doctor, who will advise them on suitable alternative medicines. Those who have appointments after July 1 "will be contacted for an earlier consultation and/or medication review".

The HSA said these tablets contain a losartan ingredient made by Indian pharmaceutical company Hetero Labs that showed trace amounts of the impurity.

According to the Food and Drug Administration of the United States, the company has been recalling certain batches of these medicines since last month.

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The HSA said that several losartan drugs have also been recalled in other countries in recent weeks.

It decided to test all losartan products here and identified that three of the 10 brands on sale here were affected.

It has also tested other drugs within the same class of hypertensive medication, such as valsartan, which has been recalled in some foreign markets. They were found to be safe.

The statement added: "HSA will require companies to make the necessary changes to their manufacturing process to ensure that the medicines do not contain these impurities in future."

The MOH said: "The public healthcare sector has made additional orders of unaffected brands of losartan.

"The additional supplies will arrive progressively from a few weeks' time. Importers will also be setting aside additional supplies for private healthcare providers."

Public sector patients will be refunded if they returned the affected medicine. Replacement medicines "during the interim period" will be charged at the same or lower price.

The MOH added: "Charges incurred for services such as additional consultations or tests to assess suitability for a switch in medicine will also be waived."

Correction note: The story was edited to correct the spelling of the drug Hyperten.