Patient A got the Implanon® implant (etonorgestrel) inserted to prevent pregnancy about two years ago. Recently, she was diagnosed with HIV and started on TDF+FTC+EFV. A few months after starting ARVs she came to the clinic and told the nurse that she hadn’t had her normal menstrual cycle. They did a pregnancy test and she was found to be pregnant? How is this possible?

The nurse phoned the hotline, and we explained to her that there’s an interaction between efavirenz and the etonogestrel implant. Efavirenz is an enzyme inducer, which means that the etonogestrel is metabolised more rapidly, so there is less of the etonogestrel in the body, leading to contraceptive failure.

Remember other enzyme inducers like rifampicin, phenytoin, carbamazepine and phenobarbitone may all affect the contraceptive efficacy of the implant!

The National HIV & TB HCW Hotline (run by the MIC) can be consulted to check for interactions on 0800 212 506. We also have an app for Android devices which includes drug interactions with HIV and TB medications. Download it at the Google Play Store.