HONG KONG, Nov 18 (Reuters) - Indigo naturalis, a dark blue plant used in traditional Chinese medicine, appears to be effective in treating psoriasis, a study in Taiwan has found.

Psoriasis is a chronic skin disease for which no cure is known, though some therapies bring about a remission. It causes red scaly patches, or plaques, which take on a silvery-white appearance and often occur on the arms, elbows, knees and legs.

A study of the findings of a clinical trial involving 42 patients who had had the condition for at least two years was published in the latest issue of Archives of Dermatology.

The researchers found that indigo naturalis in the form of an ointment was safe and effective in treating psoriasis.

"Current steroid-based medication may cause side effects like thinning of the skin, but this (indigo naturalis) has much less side effects," lead researcher Yin-Ku Lin of Chang Gung Memorial Hospital and Chang Gung University in Taoyuan, Taiwan, told Reuters by telephone.

None of the patients in the trial had serious adverse effects, though some experienced a mild skin allergy.

They applied indigo naturalis ointment on one side of their bodies and a placebo, or non-medicated, ointment on the other.

Doctors checked on their condition at the start of the treatment and after two, four, six, eight, 10 and 12 weeks.

"The indigo naturalis ointment-treated lesions showed an 81 percent improvement, the (non-medicated) ointment-treated lesions showed a 26 percent improvement," the authors wrote.

For 25 of the patients, plaques that were treated with the indigo were completely or nearly completely cleared.

Indigo naturalis has long been used, externally or ingested, to treat various infections and inflammatory diseases in China and Taiwan, such as mumps, pharyngitis and eczema.

Long-term systemic use has been linked to irritation of the gastrointestinal tract and liver problems, the researchers said. They called for more studies on ways of improving absorption of the ointment. (Reporting by Tan Ee Lyn; editing by Tim Pearce)

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