Oh, they prevent pregnancy all right - I was on them for about 30 years. But my weight went up and down like a yo-yo every month, along with my mood (not connected with the weight, though I wasn't happy about it - I would be touchy and cry like the world was ending for no apparent reason once a month - the things really messed with my head)! I was SO happy to hit menopause and dropped those things like a little packet of one-ton bricks! I really, really hope they come up with something less toxic for future women!