Can shavegrass tea make birth control pill less effective?

Yes: While Shavegrass (Horsetail) tea does not directly interfere with oral contraceptives, the mild detoxifying effects may raise the levels of progesterone in your body and cause you to ovulate even when on oral contraceptives. So indirectly you may be fighting against the contraceptive effects of your pills. Use an alternative form of birth control while doing your cleanse.