I am not sure what percentage it is decreased. Here is the warning from the information I found at
www.webmd.com:
DRUG INTERACTIONS:
This drug should not be used with the following medications because very serious interactions may occur: troleandomycin.
If you are currently using any of these medications, tell your doctor or pharmacist before starting this birth control pill.
Certain drugs can decrease the effectiveness of combination-type birth control pills by decreasing the amount of birth control pill hormones in your system (impaired enterohepatic recirculation or hepatic induction). This can result in pregnancy. Consult your doctor or pharmacist for details.
Drugs that may cause this effect include: many antibiotics (e.g., cephalosporins, chloramphenicol, macrolides, penicillins, tetracyclines, sulfas), aprepitant, bexarotene, bosentan, dapsone, griseofulvin, certain HIV protease inhibitors (e.g., amprenavir, nelfinavir, ritonavir), modafinil, nevirapine, rifamycins (e.g., rifampin), many seizure medications (e.g., barbiturates, carbamazepine, phenytoin, primidone, topiramate), St. John's wort.
Ask your doctor if you should use additional reliable birth control methods while taking any of the drugs mentioned above while also taking birth control pills.
Before using this medication, tell your doctor or pharmacist of all prescription and nonprescription products you may use, especially of: cyclosporine, corticosteroids (e.g., prednisolone), theophylline, beta-blockers (e.g., metoprolol), thyroid hormones, temazepam, aspirin, morphine, clofibric acid, acetaminophen, ascorbic acid (vitamin C), atorvastatin, azole antifungals (e.g., itraconazole, ketoconazole).
This product can affect the results of certain lab tests (e.g., thyroid). Inform all laboratory personnel that you use this drug.
Do not start or stop any medicine without doctor or pharmacist approval.
And I found an article at teenwire.com that may answer the "do I ovulate when I am on birth control?" question:
The pill contains hormones similar to the estrogen and progesterone made by a woman's ovaries. These hormones work to prevent the ovaries from releasing an egg (ovulation). They can also thicken cervical mucus to prevent sperm from joining with an egg. The ring and the patch work in the same ways. Combined hormone pills are more likely to prevent ovulation. Progestin-only pills are more likely to thicken cervical mucus.
I have read that although the pill is designed to prevent ovulation, that some women may still have "breakthrough" ovulation and that is why, in your case, you should cover your bases by using a back-up method of birth control.
I hope I have answered your questions! Please click the "Accept" button if you are satisfied with my response. If you have any further questions, please feel free to ask.
Regards,
Angela
