Two new experimental multiple sclerosis medications appear to reduce relapses and deterioration in patients -- but both carry significant side effects that may discourage patients from using them.

MS is marked by the body's immune system attacking the central nervous system, leading to weakness and disability. Though there is no cure, steroids can reduce the duration and severity of symptoms, and seven injectable medications, including ones using beta interferon, have shown success in reducing symptom relapses.

The two new medications, cladribine and fingolimod, both work by targetting different aspects of the biology of T-cells, which are key players in the autoimmune attack. The research found that patients on the medications were about half as likely to suffer relapses as those who took placebo pills or an already commonly medication.

They both come in pill form, which would make them more appealing to patients than current injectable therapies. But the researchers also found both drugs significantly lowered immune defenses. In the case of two patients, they developed herpes infections that proved fatal, including one who died of encephalopathy.

Three studies on the medications appear in this week's issue of the New England Journal of Medicine: one looking at cladribine versus placebo, one studying fingolimod versus placebo, and one comparing fingolimod versus injections of the commonly used medication Avonex, a form of interferon.

In the first study, 1,300 MS patients were given either placebo pills or cladribine, a drug already marketed for blood cancers under the name Leustatin. After two years, patients on the drug were half as likely to suffer relapse as those on the placebo, and were 30 per cent less likely to have worsening disability.

However, 20 to 30 per cent of the cladribine patients developed low counts of infection-fighting white blood cells, compared to just two percent of the others. Twenty cladribine patients suffered herpes infections versus none in the placebo group. Overall, serious problems affected almost per cent of patients taking cladribine

In the second study, about 1,000 patients were given either placebo pills or fingolimod. Only 17 per cent of fingolimod patients had worsening disabilities from MS after three months, compared to 24 per cent in those on the placebo.

Herpes infections were about the same in the fingolimod and placebo groups, but respiratory infections like bronchitis and pneumonia were nearly twice as common in the fingolimod patients.

And in the final study, 1,200 patients tested fingolimod against shots of Avonex for one year. About 20 per cent of patients on the pill had relapses versus 30 per cent on the dummy pills. As well, those taking fingolimod had less brain shrinkage -- a measure of progression of the disease.

But again, the side effects were worrying. Two people on fingolimod died of herpes infections; six had eye swelling and eight developed skin cancers. Overall, five to 10 per cent of patients taking fingolimod developed side effects.

Both medications were tested at two dosage levels. Higher doses worked slightly better for fingolimod, but patients were also more likely to get side effects with higher doses.

All three studies were funded by the pill manufacturers: Novartis is developing fingolimod, and Merck Serono manufactures cladribine.