PEP, or Post-Exposure Prophylaxis (prevention) is the use of combination anti-HIV medications after an exposure to HIV. Giving head is extemely low risk in terms of HIV transmission, and we do not generally recommend PEP for people who perform oral sex on someone else, unless in very specific situations - for instance the person giving head has bleeding gums, recent dental work or open sores in the mouth. Receiving oral sex (i.e. having someone blow you) is an even lower risk activity.