There have been reports of severe reactions and fatalities from the drug. It seems to be widely used for infections of any type these days despite being developed and marketed specifically for skin infections. Whether the reports of fatalities can be 100% directly related to the Convenia in a formal scientific manner or not I don't know. Nor do I know what the statistics are for adverse reactions.
I do however have a major concern about the inappropriate use of antibiotics and often it seems this drug is being used as a matter of convenience with little if any consideration of whether it is the optimal treatment for the condition being treated. Ideally a culture would be done before any antibiotic is prescribed however it seems rare for human physicians much less veterinarians to bother going to those lengths to ensure they treat the appropriate offending organism. They use a broad spectrum antibiotic and hope it works and Convenia is broad spectrum and convenient.
If you have an allergic reaction to a pill or liquid medication you can simply stop taking it, treat the allergic reaction symptomatically and if need be flush the drug out of the system because with a normal antibiotic, you have to keep taking it possibly 1+ times per day over a certain number of days to keep the necessary amount of active drug in the system. If an allergic reaction or severe side effect occurs, you discontinue the drug and it's out of the system in a relatively short period of time during which supportive treatment can be provided to deal with the adverse reaction.
The problem is that Convenia was developed to be long lasting and as such it is not totally eliminated from the body for a month or more (can't recall exact time frame). Since the drug is formulated to remain in the system for a prolonged period there is no way to get it out of the system quickly if there is a severe adverse reaction to it.
One of my cats was given the drug before I was aware of the potential issues and there was no problem at all. If there were a lot of animals suffering from adverse effects, I'm sure the vets would have stopped using it ages ago. That said, I am not willing to take a chance on it again unless it is the best and only option knowing it takes a month or more to be cleared from the body, because I might not be so lucky next time.
My point here is that if Convenia didn't clear the infection the first time, it seems pointless to try it again. Repeating a failed treatment doesn't make sense under any circumstances but particularly with a drug that cannot be withdrawn once given should an adverse reaction occur.