Can i take rocephin (ceftriaxone) if i'm allergic to penicillin?

Very likely: There's a very small, less than 1% chance of cross-reaction between penicillin and a cephalosporins. If you had a severe reaction to penicillin, you're more likely to have a severe allergic reaction to anything else. However, I would not hesitate to take ceftriaxone if it is the appropriate medication

Depends.: It depends how severe your reaction is and who the prescribing practitioner is. There is about a 10% cross reactivity with cephalosporins (Rocephin (ceftriaxone)) and penicillin based drugs. Usually if the reaction is minor, I will prescribe a cephalosporin like Rocephin (ceftriaxone). However, if the reaction to the penicillin based med is severe like shortness of breath, swelling or anaphylaxis I may avoid them completely.