Effectiveness depends on correct use. If 100 girls use it with great care for a year only 1 will become pregnant. But in actuality 2 or 3 out of 100 become pregnant every year. So if you use only oral contraceptives, accuracy is asked for.
Reliability decreases if:
you forget to take one or more pills. There may never be more than 36 hours between two pills.
you stop longer than a week before starting a new package.
you suffer from severe diarrhoea. Keep taking your pills, but also use condoms. When you have recovered, keep using condoms for 7 days.
you have to vomit within an hour after taking a pill. Take another pill from a reserve package.
you take medication that affect efficacy, like antibiotics, norit, antiseizure and anti- fungal medications. If you get a prescription always ask if it affects the efficacy of the pill. The doctor may forget to warn you. If it interferes you have to use condoms while you take the medicine and often continue using these for one or two weeks.