Ideally,birth control pill is 99% effective,but certain factors can reduce the pill to about 91%,and this is the reality because women are not perfect and consistent in talking birth control pills,that means 9 out of every 100 persons using pill get pregnant each year.
Not taking the pills everyday is the main factor that makes birth control pill to be less effective.
Other factors that affect the effectiveness of birth control pill includes;being overweight,taking some form of antibiotics example; Rifampin, taking antifungal drug like griseofulvin, taking some form of anti-seizure medications, certain HIV medications, and having diarrhoea or vomiting for more than 48hours(2days),