Vets should warn you if there is a ssrious side effect, especially when an injection cos the effects cant be stopped once given.
However in their defence all meds can have side effects but most dont with most cats so they would be worrying people unnecessarily.
The only way this can be sorted is if every owner asks about side effects before any meds or injections are given.
It should be at the top of your list when going to the vets, its always best to take a written list of question cos in the panic of the moment its easy to froget to ask stuff.
I have taken a list in the past and forgotten to ask some of the questions even...............just human response to panic, worry and stress