Eight brands of ranitidine medicines - used to treat patients with conditions like heartburn and gastric ulcer - were removed from Singapore shelves for containing a cancer-causing agent.

The Health Sciences Authority (HSA) said the medicines were found to contain a nitrosamine impurity, N-nitrosodimethylamine, in excess of the internationally acceptable level.

HSA's order to stop the sale of these ranitidine medicines comes amid similar moves worldwide, after recent medical studies found that prolonged exposure to doses of nitrosamine impurities that are much higher than levels people are usually exposed to could cause cancer in animals.

Nitrosamines are environmental contaminants and can be found in food or the environment in minute amounts. For example, they can be found in processed food such as bacon and sausages.

Nitrosamine impurities have also been found to form unexpectedly during the manufacture of some medicines.

The eight brands for which sales have been stopped - and supplies to clinics, hospitals and pharmacies terminated - are Aciloc, Apo-Ranitidine, Hyzan, Neoceptin, Vesyca (film-coated), Xanidine, Zantac (injection, syrup and tablet) and Zynol-150.

In its update on its website on Monday, HSA noted that this was a precautionary measure.

The authority said patients who have been prescribed the affected medicines for short-term use can continue to use them. This is because the international criterion used to assess whether an affected medicine is safe or not is based on its effects if a person were to take it every day for 70 years.

Generally, ranitidine is prescribed for short-term use of a few days to a few weeks, and used only when necessary.

HSA added that there are other medicines that can be used as alternatives, and patients who have questions can speak to their doctor or pharmacist.

The authority added that it is working with the companies supplying these medicines and international regulatory agencies to verify the causes of the nitrosamine contamination, and to identify the necessary measures to address the issue.