Health officials have issued a new warning to people taking herbal products, saying their health may be at risk from products which contain undeclared prescription medicines.

Four different products imported from Asia - being sold as herbal medicines - have tested positive for erectile dysfunction and weight loss prescription medicines following routine investigations by the Ministry of Health Medicines Safety Authority, Medsafe.

Acting Director-General of Health, Debbie Chin, said it was a breach of the Medicines Act 1981 to sell or supply herbal products which contained undeclared prescription medicines.

"There is a real potential for harm to occur when prescription medicines are used by consumers without a prescription, because of the absence of any medical supervision," Mrs Chin said.

If people were in doubt about the contents of any herbal product they were taking for a health condition, they should seek advice from a health professional, she said.

Medsafe investigators located the four products imported from Asia through a retail outlet's advertisement for "herbal Viagra".

The investigation also uncovered a quantity of medicines labelled as containing prescription medicine, which were not approved for supply in New Zealand.

Testing of the products Meng Rong and an unnamed tablet with markings 'VG' showed that they contained the prescription medicine sildenafil, prescribed for the treatment of erectile dysfunction.

The herbal product Yixinjiaonang contained the prescription medicine tadalafil, also prescribed as an erectile dysfunction treatment.

"Sildenafil and tadalafil are known to interfere with some heart medications and their use could be fatal to some individuals."

Tests on the fourth product, Reduce Weight, showed that it contained the prescription medicine sibutramine used for the treatment of some overweight (obese) patients.

"Sibutramine can cause increased blood pressure and heart rate and cannot safely be taken by a range of people, including those with glaucoma, mental illness and severe liver or kidney problems."

Ms Chin said it should not be used in combination with other medicines such as some anti-depressants and migraine treatments.

"Consumers should immediately stop taking these products and seek medical advice from their doctor if they are taking other medicines, felt unwell after taking the products, or if they felt unwell after they stopped taking the products."

Medsafe investigations are continuing and prosecution is being considered against the alleged distributor and all known stocks of these four herbal products have been seized.

"We are concerned about the continuing numbers of so-called herbal medicines found to contain undeclared prescription medicines."

The warning comes after Health Director-General Karen Poutasi warned people in March to immediately stop taking two herbal products which contain undeclared prescription medicines and could be dangerous or fatal in some cases.