Is this an allergic reaction?

If I got an allergic reaction from Cephalexin, does that mean I am also allergic to Penicillin?

• I was taking Cephalexin for an infection I had. After a few days I developed a bad all over body itchiness. They said it was an allergic reaction. I know if you are allergic to certain antibiotic, you are also allergic to others. Because I am allergic to Cephalexin, does that mean I'm allergic to any other ones, like Penicillin?

• Answer:

  I often run into patients with a penicillin allergy. There's about a 10% chance of him/her reacting to a drug of the cephalexin class. I've never really thought about it the other way around. Since you had what sounds like a classic allergic reaction to cephalexin, I'd avoid penicillin if possible just to be safe.