I'm also allergic to penicillin (among a couple other antibiotics) and have not had a problem at all, and was cleared for all trades after my medical. Just make sure, as was mentioned, to have a medic alert bracelet, and be sure to mention it anytime you go in to see a doctor, medic or other health care professional. While it should be on your documents, and they should always ask, mistakes can happen, and you want to be sure. One time, a doctor handed me a prescription for penicillin, even though the allergy was written in bright red at the top of my file. Just make sure to look after yourself.