The main purpose of oral contraceptives is to prevent pregnancy. Chemical equivalents of the female hormones, estrogen and progesterone, work to suppress ovulation, thicken cervical mucus to create a hostile environment for sperm and thin the uterine lining to block implantation if an egg should be fertilized. Theoretically, any drug that stimulates the production of liver enzymes can decrease the effectiveness of oral contraceptives.
Rifampin
Antibiotics are prescribed to treat bacterial infections. One particular antibiotic, rifampin, has been demonstrated to decrease the effectiveness of oral contraceptives. A review of medical literature suggests that antibiotics decrease the concentration of the hormones thus decreasing the effectiveness to prevent pregnancy. Rifampin is indicated in the treatment of tuberculosis and asymptomatic carriers of the meningococcal bacteria. According to RxList, this drug is known to induce certain liver enzymes called cytochrome P-450 enzymes. The therapeutic blood levels of drugs, such as oral contraceptives, that are metabolized by these enzymes will be altered if taken with rifampin.
Amoxicillin
Amoxicillin is an antibiotic commonly prescribed to treat bronchitis, urinary tract infections and Lyme disease. Dentists prescribe amoxicillin as prophylaxis against endocarditis in susceptible patients prior to oral dental procedures. It is controversial whether amoxicillin decreases the effectiveness of oral contraceptives. However, until conclusive data settle the issue, some clinicians recommend that women use alternate forms of contraception for the duration of treatment with amoxicillin.
Barbiturates
Barbiturates are central nervous system depressants capable of producing mild sedation, reducing central nervous system synaptic transmission and causing a sedative-hypnotic effect. Physicians prescribe barbiturates to treat insomnia, symptoms of anxiety, and seizures. Barbiturates, especially phenobarbital, induce the production of liver enzymes and increase the clearance time of drugs that are metabolized by the liver. Barbiturates can accelerate the metabolism of estrogen and progestin found in oral contraceptives thus decreasing their ability to prevent pregnancy.
Griseofulvin
Griseofulvin is an antifungal medication that inhibits the growth of the type of fungus that causes ringworm infections. It is used to treat athlete's foot, jock itch and fungal infections of the hair, nails and skin. This drug may decrease the effectiveness of oral contraceptives, resulting in an unplanned pregnancy. Health care professionals advise the use of an alternate form of birth control during the duration of treatment with griseofulvin.
