Here we go:

From this website : www.eff.org/Net_culture/Misc/technoculture.paper

I got the following info: (an interviewer talking to a NASA scientitst (supposedly))

(2) About menstruation in space, he said "Even if there were a problem - and there isn't - there still wouldn't be a problem." Not only can flow be arrested with pills or endometrial fibulation (extraction), but the imagined problems of retrograde (backflowing) menstruation has never been found to have any consequences.

"So do they just bring tampons along on the space flights?" I asked.

"Basically, yes," he said. (He referred me to an article he wrote on the reproductive functions of men and women in the space environment in Obstetrical and Gynecological Survey, vol 45, #1, p. 7. (1989)