Any birth control pill should be effective without another means of protection as long as you have taken it for a week straight before having unprotected sex and you take it right (at the same time every day). No matter how high the dose is, they are equally effective.

But no pill is 100% effective... they're about 97% effective. if you are really worried about it, use a condom. But that's the reason we take the pill, so we don't have to use condoms... right?