Hi, My Husband Started Taking Cephalexin On This Past Wednesday.

Hi, my husband started taking Cephalexin on this past Wednesday. That night as he was driving to work he started having racing heart (120bpm) and numbness that started in his left arm and moved to his right arm. Since that night, it has happened 5 other times, this last time being the worse with the numbness spreading to his neck and head as well. He went to the ER where they hooked him up to the EKG and ran full blood work. They told him all tests came back normal, there is no blockage anywhere and he seems like a typical healthy young man. They gave him no answer.
My question is, could he be having an allergic reaction or is this a more serious side effect of this drug?
His last dose is tomorrow night and he says hes never been allergic to penicillin type drugs before.