Certain medicines can make the implant less effective against pregnancy. For example, medications used to treat epilepsy, HIV or tuberculosis can have this effect, as can remedies like St. John's wort and certain antibiotics. If you're taking any of these medications, it's recommended that you use an additional type of contraception or a different kind of contraception that's not affected by medication. Your doctor will also be able to guide you on what is and is not affected by Nexplanon.