The combined or phasic pill is very efficient (92-99.7%) if taken within six days after periods or abortion.
If it is taken at any other time it requires a period of one month to become effective. Failure rate is very negligible in the order of, 0.05-0.08%. Its efficiency is reduced when it is not consumed on a regular basis or if taken in combination with other medications like certain antibiotics and drugs for seizures, migraine disorder, and anti-TB drugs. The herbal supplement, St. John's wort also interferes with the functioning of The Pill. The birth control pills do not protect a woman against sexually transmitted (STDs).
- Birth Control Pill - (http://kidshealth.org/teen/sexual_health/contraception/contraception_birth.html)