ANN ARBOR, Mich., June 17 (UPI) -- The oral use of RU-486's companion drug misoprostol is safe but vaginal use may undermine the body's immune response, U.S. researchers suggest.

University of Michigan scientists says that the drug misoprostol is U.S. Food and Drug Administration-approved to be taken by mouth along with RU-486 to end a pregnancy.

However, many women have received the drug vaginally as part of the two-drug combination, a method of delivery not evaluated by the FDA, Dr. David Aronoff says.

The study, published online ahead of print in the Journal of Immunology, finds that in animal and cell culture studies misoprostol -- when given directly in the reproductive tract -- suppresses key immune responses and can allow a normally non-threatening bacterium, Clostridium sordellii, to cause deadly infection. When absorbed through the stomach, the drug didn't compromise immune defenses or cause illness, the study said.

"The results provide evidence why doctors should avoid giving misoprostol vaginally and underscore the wisdom of giving it by mouth instead," Aronoff says.

The off-label use of misoprostol drug may be responsible for a handful of rare, fatal infections seen in women taking the drugs since 2000, Aronoff says.