My oldest and youngest both presented with what we were told was serum/serum like sickness in response to Ceclor. I realized my youngest was having the same sort of reaction when it occurred, so her response didn't get to the level that my oldest's did.
He presented with tiny hives to start, taken off the med, told to give him Benadryl, did that, hives came back. We were right in the middle of moving, so he was with my inlaws for the day. Everything seemed fine, then his joints started to swell, and when my mother in law called to say she thought maybe he needed to be seen in emerg, he had smallish red dots (petechiae? is that right?) and by the time we got out to their acreage, they were large - he looked like someone had worked him over with a baseball bat. The color reminded me of port wine stains. He also had trace amounts of blood in his urine.
We were living in far northern Alberta at the time, so the docs consulted with docs at the UofA over the phone, and they treated with steroids. They were talking about sending him out by plane but the docs in Edmonton wanted to see how he responded to the steroids. We had to take urine samples to the hospital every day for a week or two after all of this, to make sure the blood trace was clearing up.
I figured out what was going on with my daughter pretty quickly, when she reacted, so her response wasn't like my son's, as I said earlier. Swollen joints, tiny hives, etc.
We just say our other son is allergic to it.
We didn't see any point in finding out if he is. My kids are weird.
Now, from my understanding, Ceclor is rarely ever prescribed anymore, because so many people had issues with it. My oldest is 22, and this was about 20 years ago.