Estriol is a female sex hormone that surges in a woman's body during pregnancy. Studies have shown that women with MS have significantly decreased relapse rates during the third trimester of pregnancy, when estriol levels are most elevated, and that relapse rates rebound after delivery, when estriol levels drop abruptly.

In one trial, nonpregnant women treated with pregnancy levels of oral estriol had a significant decrease in brain lesions after six months of treatment. When treatment stopped, lesions returned to previous levels. When treatment resumed, the lesions again decreased.

In addition to reducing relapse rates, research shows that estriol may improve cognitive function.

As of yet, estriol is not approved for use in the U.S. Trials are currently underway to test the efficacy of estriol, which has the potential to play a role in the treatment of MS.