A decision by the Irish Medicines Board to make St John's wort (Hypericum perforatum) a prescription-only medicine from January 1, 2000, is to be challenged in the courts.
St John's wort is used as a herbal remedy to treat mild depression.
The board's decision is based on concerns raised during a review of data from applications for authorisation of new products containing St John's wort.
A spokesman said that it was not clear how St John's wort achieved its antidepressant effect, but its was reported to act as a monoamine oxidase inhibitor. The medicines board had been concerned that reactions to foods, such as red wine or cheese, that could prompt a hypertensive crisis if taken in conjunction with MAOIs was possible.
He added that patients with mild to moderate depression should be under medical supervision.
"Self-diagnosis and self-medication are inappropriate," he said.
The decision has outraged supporters and practitioners of alternative medicine and the health food industry. A petition of more than 30,000 signatures is to be presented to the Irish health minister (Mr Brian Cowan) calling on him to reverse the board's decision.
The Irish Health Producers' Alliance, an umbrella body covering manufacturers, wholesalers, retailers and medical herbalists, has taken legal advice and is planning to challenge the decision in the High Court in Dublin if the minister does not set the board's decision aside.
The Irish Green Party is also organising opposition to the move.
Mr Trevor Sargent, a Green member of the Irish Parliament, said that St John's wort was freely available throughout Europe and the United States and that the only beneficiaries of the restriction would be the pharmaceutical companies.