Can i take cephalexin if i am highly allergic to penicillin? i have not taken penicillin since i was very young. reaction of hives convulsions

Penicillin allergy: You have what is known as an immediate hypersensitivity reaction to penicillin derivatives and should never be given one, since this is life-threatening. There is said to be cross-hypersensitivity reactions to cephalosporins in 5% of pen allergic persons, and 0.1% of these have a severe reaction. My advice would be not to take a cephalosporin antibiotic unless there was no alternative.