Timing is definitely the key. The higher dose pills and lower dose pills are both 99% effective as long as they are taken on time. There are studies showing that the number goes down with low dose pills slightly for obese women, however, so if you are overweight maybe you should consider not using a low dose pill. Other than that, there is nothing to worry about and all kinds of pills are VERY effective when taken correctly. Just make sure to wait (usually 2 weeks, but depends on the pill...read your packet) before having sex without a condom/another form of protection when you first start the pill!