Most Helpful Guy
They are if used right, and if used at all. Help your guy put it on. This will ensure it goes on right, AND that he maintains his erection. The logistics of putting on a condom can make an inexperienced dude go soft. :) Suck his cock with the condom on for a bit if he's lost his erection somewhat. Or at least pump it slowly and seductively with your hand. :)
The guy has to be extra careful if condoms are your birth control method. I've never seen one actually break but it happens. They're more likely to just slip off if his cock is wet (from whatever). Feel down between your legs to check it if you think you need to, to make sure it's still on before he cums. :) And good luck.
Most Helpful Girl
If you use condoms correctly then it's just as effective.
by the way the pill won't destroy your cycle. I've been on it for 2 and a half years and I get my period every month the same time.