I know its not 100% but for the small percentage, what are the reasons for it?
I would recomend you use birth control and condoms so nothing happens, but like me and my boyfriend I dont have access to birth control without parental consent until next month, so if you have access get it, but if not, just be very careful with only condoms, check it every once in a while to make sure it didn't break or slip off inside you, and make sure your well lubed so breaking wont happen. We trust trojan, but with condoms when it is put on just make sure you dont get a bubble in the top or it can pop your condom keep all air out and if you use ecstacy, make sure you leave a little tip at the top as a resevoir because they dont come with it for pleasure reasons, but its safer to make one anyways.
there a small chance. its 99% effective but theres always he wore one that was too big or too small.. or it was broken or he broke it... so yes it can be risky to just use a condom.
but regardless of what you think I would use condoms along with the birth control. because birth control doesnt protect you from stds & a condom does for some.
The reasons it is not 100%? Because there are few things in life that work 100% of the time. It could rip or break, it could be put on wrong, it could come out, it could have a hole, etc etc etc. You're likely to be fine.
How safe depends on a lot of factors...bottom line, you're taking a risk.