Migraine sufferers have been urged not to take a Chinese herbal medicine used to treat the severe headaches because it contains a "potentially toxic" ingredient, experts said.
Zheng Tian Wan herbal pills contain aconite, a poisonous plant which can be toxic to the heart and the nervous system and can cause death, said the Medicines and Healthcare products Regulatory Agency (MHRA).
Aconite, also known as monkshood, is on a UK list of restricted herbal ingredients, a spokesman said.
He said the sale of the pills came to the attention of the MHRA after a herbal practitioner in the West Midlands supplied them to a patient.
The agency has received reports about three patients who suffered suspected side effects after taking products that contained the ingredient.
The spokesman said that one patient suffered kidney problems, a second person was hospitalised after becoming dizzy and suffering from pins and needles and the third experienced palpitations, aches and pains with shortness of breath but recovered after they stopped taking the product.
He added: "Aconite should not be used in unlicensed products for oral use and Herbal practitioners in the UK should only use aconite in preparations for external use only on unbroken skin.
"It can be prescribed in oral medicines by a qualified medical doctor under supervision as a prescription-only medicine. The product is an unlicensed herbal medicine which means it has not been assessed for safety and quality by the MHRA."
Andrea Farmer, herbal policy manager at the MHRA, said: "Herbal medicines can have a very significant effect on the body. In certain circumstances, such as with aconite taken orally, they can be extremely dangerous.
"Products intended for oral use containing aconite are not permitted in the UK without appropriate authorisation.
"If you have taken Zheng Tian Wan or any other aconite-containing product, speak to your GP or healthcare professional as soon as possible."