Contraceptive failure with etonogestrel implants and efavirenz: case reports
Two case reports of unintended pregnancies suggest that etonogestrel implants should be used with caution in patients on efavirenz.
The first case had been receiving efavirenz, zidovudine and lamivudine since November 2002 and had an etonogestrel implant inserted in January 2004. Pregnancy was detected in April 2006 and conception dated to December 2005. The second case had an etonogestrel implant inserted in July 2005 and started efavirenz, tenofovir and emtricitabine in April 2007: in October 2007 she became pregnant following a condom rupture. In both cases, implants were removed at pregnancy diagnosis and were found to have been properly inserted.
The most probable mechanism explaining the contraceptive failure is low etonogestrel concentrations due to induction of cytochrome P450 by efavirenz. As it is not currently possible to predict the decrease of etonogestrel implant efficacy when an enzyme inducing drug is coadministered, an alternative method of contraception should be recommended.
Source: hiv-druginteractions.org (06 December 2011).
Leticee N et al. Contraceptive failure of etonogestrel implant in patients treated with antiretrovirals including efavirenz. Contraception, 2011, epub ahead of print