Everything requires energy, and magic is expending it, or at least redirecting it elsewhere. We can say the same amount of energy exists before as after, and we’ve repurposed it, but this is a philosophical subject. We care more about the cost to the practitioner here. Those who suffer few or no effects of performing magic are almost gods, but mortals aren’t so lucky. Adding a cost to wizardry is an easy way to limit their powers.
The obvious and default answer, which no one will feel is a cliché, is that it’s physically draining to perform wizardry. Everything else is, so why wouldn’t it be? We also can decide that using it is like alcohol, which is a depressant but, in the short term, acts as a stimulant instead; people can feel energized during a magical battle but “crash” when it’s over, needing a deep sleep if they expended a lot of energy; use an analogy of your choice with different side effects to invent something original. We can decide that magic ages the practitioner prematurely, or reduces their ability to have children due to exposure to the energy, as does radiation. An old idea is that a spell needs to be memorized and, once cast, the wizard forgets it and must relearn it; what if casting it affects the mind in other ways? Maybe the spell isn’t the only thing they forget. Or perhaps magic makes them have nightmares or slowly go crazy. Just be sure to invent a price.
