The medication regimens for PEP and PrEP are similar, but usually a PEP regimen contains 3 medications active against HIV while PrEP is 2 medications (combined into one pill). If you've been taking your PrEP regularly, there is no need to add a 3rd medication. PrEP alone is extremely effective at preventing HIV. If you've been missing PrEP doses, you should talk to a provider about whether it makes sense for you to add a 3rd drug for PEP.