IT'S common knowledge that certain drugs just don't go well together, and when those drugs affect your contraceptive's efficacy, the baby blockers become useless.

Therefore, women should be fully aware of drugs that can interfere with birth control and take the necessary precautions to avoid taking them where possible, or use added protection like a condom as a backup during intercourse. Otherwise you could find yourself pregnant even while you think you're protected.

Professor Horace Fletcher, gynaecologist and obstetrician at the University Hospital of the West Indies said, for example, that there are drugs that can interfere with birth control pills.

"Many drugs affect the metabolism of birth control pills. Simply put, when the pills are digested they are absorbed into the blood and the liver enzymes break down the chemicals in the pill releasing active substances. Some drugs like phenobarbitone (used to treat epilepsy) enhance the activity of liver enzymes and increase breakdown," he said.

He explained that when the drug is excreted from the liver into the gut, the bacteria in the gut breaks the drug down further and it is once again absorbed and again passed into the liver.

"If the bacteria (natural commensals) are removed such as with the use of antibiotics, less active drugs will be available as the second pass mechanism is lost," he said.

"Other ways of blocking the active drug is to prevent transport by the proteins in the blood (sex hormone binding globulin) or to block the receptor where the active drug should work. Some phytoestrogens can do this (maybe soy products)," he said.

Pharmacist Paul Pearson from Self Plus Pharmacy on Brunswick Avenue in Spanish Town said antibiotics listed below can all cause a decrease in the efficacy of oral contraceptives.

Rifampin: used to treat tuberculosis and bacterial infections.

Tetracycline: often used to treat acne

Ampicillin and Amoxicillin: used to treat bacterial infections such as UTIs.

Pearson said the anti-seizure and migraine medications that can interfere with oral contraceptives are:

Phenobarbital: used to treat seizures and sometimes insomnia.

Carbamazepine: used to treat epilepsy, bipolar disorder, and sometimes ADHD and schizophrenia

Topamax (active ingredient Topiramate): used to treat migraines and epilepsy

He said when it comes to natural products, there are no known studies showing any activity towards contraceptives.