An extract of the herbal remedy St. John's wort is as effective as a commonly prescribed drug for people with moderate-to-severe depression, researchers reported.

They compared the extract called WS 5570, which is made by the German company Dr. Willmar Schwabe Pharmaceuticals, and the antidepressant paroxetine sold by GlaxoSmithKline Plc under the brand name Paxil or Seroxat.

St. John's wort is also known as hypericum perforatum.

"In the treatment of moderate to severe major depression, hypericum extract WS 5570 is at least as effective as paroxetine and is better tolerated," Professor Armin Szegedi of the Charite-Universitatsmedizin, part of the University of Berlin, said in a report published online by the British Medical Journal.

In a study funded by the German company, the researchers compared the treatments in 251 patients, between 18 and 70 years of age, for six weeks. Half of the patients taking the extract reported an improvement, compared with one third taking paroxetine.

Patients taking the extract also reported fewer side effects, such as stomach disorders, according to the researchers.

Paroxetine belongs to a class of drugs known as selective serotonin re-uptake inhibitors (SSRIs). British and European health authorities have voiced concern that these antidepressants may increase the risk of suicide in young people.

"Our results support the use of hypericum extract WS 5570 as an alternative to standard antidepressants in moderate-to-severe depression, especially as it is well tolerated," Szegedi added.

St. John's wort has been called nature's alternative to Prozac, but researchers have warned that it can interfere with hormone treatment, antibiotics and chemotherapy.

Earlier studies have shown the herbal remedy is also effective for treating patients with mild depression.

The researchers called for further studies to confirm their findings.