The US Food and Drug Administration (FDA) has issued an alert highlighting evidence that the popular herbal remedy St John's Wort can interfere with drugs used to treat HIV-infected patients.
It has also raised the possibility of complications for other patients taking similar medication, including those undergoing heart transplants.
The move is a further blow to the campaign to make the herb, taken to relieve mild depression, available in the Republic from health food stores. Since January 1st, a medical prescription has been required - the restriction was introduced on the advice of the Irish Medicines Board.
The move by the FDA was prompted by research published in the latest issue of the Lancet medical journal. The authors of the research suggest the findings add to growing concerns that the herb (known as Hypericum peforatum) may interact dangerously with prescription medicines.
An IMB spokesman said the latest studies had reinforced its view that there was cause for concern. "The IMB has previously highlighted the potential for St John's Wort to interact with a wide variety of commonly used medicines," he said.
"This new information reinforces the IMB's previous position that regulation is required to ensure the safe and appropriate use of this herbal medicine."
The FDA cited one study conducted by the National Institutes of Health in the US, which showed that for patients taking St John's Wort the effectiveness of the anti-viral drug indinavir was "dulled".
The Lancet also carries research completed by doctors at the University Hospital in Zurich suggesting the transplant drug cyclosporin - used to suppress immune systems in transplant patients who receive new organs - may have reduced effectiveness in similar circumstances, and may cause heart problems to recur.
While both studies involve low patient numbers, and certain foods may cause similar reactions, independent experts said the conclusions were significant.
The FDA is working with drug manufacturers to ensure labelling of medications are revised to highlight the potential for interactions.
The problem relates to difficulties with a particular series of biochemical reactions, known as a metabolic pathway, that occurs with certain anti-viral drugs.
As many prescription drugs used to treat conditions such as heart disease, depression, seizures and certain cancers, or to prevent conditions such as transplant rejection or pregnancy (oral contraceptives), are metabolised via this same pathway, the FDA said healthcare providers should alert patients about these potential drug interactions. which could reduce their therapeutic effect.
Serious adverse reactions should be reported to the FDA, it advised.
