A herb popular for relieving hot flashes and other symptoms of menopause may cause liver damage, experts warn.
Black cohosh has become popular in recent years
The Medicines and Healthcare products Regulatory Agency (MHRA) said all products containing black cohosh should in future carry a warning.
Two expert committees examined safety data and concluded that liver injury was rare - but could be serious.
Patients are advised to stop using black cohosh products if they develop signs suggestive of liver injury.
These can include pain on the right side of the stomach just below the ribs, unexplained nausea, flu-like symptoms, dark urine and yellowing of eyes or skin.
Professor Kent Wood, MHRA chief executive, said: "The MHRA is working with the herbal sector to ensure that labels of black cohosh products carry updated safety warnings.
"The labels will point out the possible symptoms so that appropriate action can be taken without delay."
Black cohosh has been used for many years in Europe and North America.
It gained in popularity after research linked hormone replacement therapy with a raised risk of heart attack, stroke and breast cancer.
Black cohosh is also used to treat arthritic and rheumatic pain, high blood pressure, tinnitus (ringing in the ears), asthma and whooping cough.
However, is not recommended for women who are pregnant or breastfeeding.
Dr Margaret Rees is an expert in post-menopausal health at Oxford Univeristy, and editor-in-chief of the Journal of the British Menopause Society.
She said: "There is concern about the safety of herbs because we often do not know what is the active ingredient.
"Also there is little control over the quality of products so we often don't know what is actually present in individual herbal preparations."
Dr Rees said a new EU directive should help to address this second problem.