A BATCH of Po Chai capsules has been recalled after tests here Showed it contained undeclared trace amounts of two controlled substances, one of which may be cancer-causing.

A check with five medicine halls showed no capsules on the shelves, following the recall on March 8 by the Health Sciences Authority (HSA).

Its move sparked even wider action in Hong Kong, which announced a recall two days ago of all Po Chai capsules as well as pills as a "precautionary measure".

A .spokesman for the Department of Health in Hong Kong pointed to the ''very serious potential side effects'' of phenolphthalein and sibutramine, which can hit "vulnerable groups like children and the elderly".

The traditional Chinese medicine is widely used in Singapore as a remedy for digestive upsets.

It is available in both pill and capsule form, though the latter is not popular with customers, said retailers here.

The HSA said it had no plans to widen the recall as it had tested other samples of Po Chai pills from the same manufacturer and they were found to be all right.

HSA told The Straits Times that the batch of Po Chai capsules manufactured by Li Chung Shing Tong (Holdings) in Hong Kong and imported by  Li Chung Shing Tong here contained trace amounts of phenolphthalein and sibutramine.

Phenolphthalein is used as a laxative to treat constipation and has been banned in Hong Kong for its cancer-causing effect, said the Hong Kong spokesman.

Sibutramine is an appetite suppressant used for weight loss. A possible side effect is increased blood pressure and pulse rate, which can be dangerous for people with heart problems.

The importer could not be contacted for comment.

But HSA said that consumers in Singapore who have ingested the product do not need to be alarmed. It has labelled the recall as Class 2, which means that users are unlikely to suffer serious harm.

It said that ''the amounts of these two undeclared ingredients are in low concentration" and more than 100 times lower than the amount required tor harmful effects to take place.

An HSA spokesman added: "These quantities are unlikely to pose significant safety concerns when taken on a short-term basis."

So far, the drug licensing authority has nut received any report of adverse drug reaction to the Po Chai capsules, which were recalled from retail outlets like medical halls and convenience stores.

HSA said pharmacies like Watsons and Unity were also affected.

Chinese medical hall Eu Yan Sang, which has 40 outlets islandwide, took the products off the shelves in January, after being instructed to do so by their distributor, who highlighted the two substances, said chief executive Richard Eu.

Mr Eu also said the recall would have little impact on consumers, who prefer the small reddish-brown pills by far.

The owner of Ban Chuan Tong Drug Store in Toa Payoh Lorong 4, Mr Foo Tai Toon, said he did not stock Po Chai capsules.

He said in Mandarin: "Many years ago people told me that they're not effective so we've never sold them."