I am definitely allergic to penicillin, so I wear a medical wrist bracelet all the time. If I accidentally get a dose of penicillin, I will have an allergic reaction just like some people get with bee stings or exposure to peanuts. Everything will swell up and I won't be able to breath on my own.
Thankfully, I've been able to take other prescription antibiotics whenever I get infections. The ER doctors look for those medical bracelets automatically, so even if I'm unconscious they'll know not to give me penicillin intravenously.