'There are a couple things that can make the implant less effective. These include medications for yeast infections, tuberculosis, HIV, seizures, and some mental conditions. Some herbs, like St. John's Wort, can also mess with the implant's effectiveness. Finally, there isn’t enough research yet to be completely sure that the implant is as effective in people with a body mass index (BMI) higher than 30. (Seriously, someone get on that research.)'
There aren't many things - and the medication for yeast infection is typically only one pill, so if you need to take it, simply abstain from sex for a few days or use a condom.
Also, I have a BMI of 31 and my implant has just been replaced after 3 years with no issues.
So other than that, I wouldn't worry.