I don't post often but thought it might be helpful to others to know that Cephalexin has the potential to cause a life threatening reaction in some people. I thought at first that I might be having a delayed reaction to having injections of numbing agents at the dentist. After being told that I had an infected tooth, when I came home I took a 500 mg. generic Keflex tablet. Within an hour my palms started to itch badly and next my breathing was affected severely. It really scared my husband and I so he called 911. They gave me oxygen and an iv on the way to the hospital. At the hospital, they injected benedryl and some cortisone (I think it was medrol). Then they gave me a drip of some kind of antacid. My body had swollen a lot, especially my legs and I looked like I had a bad sunburn and felt like an over-inflated balloon! I called the pharmacist today and he said because I had taken the medication and then resumed it a few weeks later that I had become sensitized to it. Who would have thought this would happen? I guess the trip to the ER was meant to be because they told me that had PVCs which are Premature Ventricular Contractions. They couldn't figure out what was causing them. They advised me to stay but I would had to sit on a guerney for hours and have many more tests(probably all night) and that would killed by back and Iknow I could not have slept. My EKG and enzmes were ok so figured I wasn't having a heart attack. I will go to see a cardiologist on July 10. I'm66 and was checked out about a month ago with an EKGthat was ok. I've bouts of squeezing pain in my upperback, arms and jaw that are newfor me. Just thought I would let others know about the possible side effects of cephalexin!!