Have a subtle chat. I'd imagine if she's sexually active at 16 she's already got her contraception sorted out, at least I would hope so, but not knowling how clued up your daughter is, it's hard t tell. to be honest she should be usuing barrier contraception as well, as the pill gives no protection against AIDS, syphilis, hepatitus etc etc etc etc, but be subtle, as at 16 she might feel it's not your business, and just be embarassed, especially if you attempt to "educate" her and she's already got it all covered, so if I were you, light general chat first to see how the land lies.