Which of the following drugs reduces efficacy of combined OCPs
**Core Concept**
The efficacy of combined Oral Contraceptive Pills (OCPs) can be influenced by various factors, including drugs that affect the cytochrome P450 enzyme system, particularly CYP3A4. This enzyme is responsible for metabolizing the hormones estrogen and progesterone, which are key components of OCPs.
**Why the Correct Answer is Right**
Drugs that induce CYP3A4 can lead to increased metabolism of the hormones, resulting in reduced serum levels and efficacy of the OCPs. Rifampicin, a well-known inducer of CYP3A4, can significantly decrease the efficacy of combined OCPs by increasing the metabolism of ethinylestradiol and levonorgestrel. This can lead to breakthrough bleeding, ovulation, and reduced contraceptive efficacy.
**Why Each Wrong Option is Incorrect**
* **Option A:** Griseofulvin is an antifungal medication that inhibits CYP2C9 and CYP3A4, but it does not significantly affect the efficacy of combined OCPs. It may even increase the levels of ethinylestradiol.
* **Option B:** Cimetidine is an H2 receptor antagonist that can inhibit CYP3A4, but its effect on combined OCPs is minimal and not clinically significant.
* **Option D:** Ethosuximide is an antiepileptic medication that can inhibit CYP3A4, but it is not known to affect the efficacy of combined OCPs.
**Clinical Pearl / High-Yield Fact**
When prescribing combined OCPs, it is essential to consider the potential interactions with other medications, particularly those that induce or inhibit CYP3A4. Patients should be advised to use alternative contraceptive methods or have their OCPs adjusted if they are taking medications that may affect their efficacy.
**Correct Answer:** C. Rifampicin