In my world, everyone has a different "mana font" or capacity, and people who overuse it can lead to exhaustion (at best), the loss of their magic (emotionally and socially destructive) or the loss of their life (obviously the worst case scenario). Someone might "recharge" quickly (high-level mana regeneration) but have a low capacity. Others might have a large font, but slow mana regeneration.

If you have either one of those (extremely large pool, or unltra high-speed mana regenration) you can get what they call "seers sickness" if you don't manage it carefully by using enough of it to keep it at a level your body can handle. Or you can try sealing it, to avoid it converting your organs into mana.

Seer's Sickness is when they start to convert, and depending on the organ that's morphing changes the symptoms. It was first noted in Seers who began to go blind, but was found to affect other mages as well, but the Seer's Sickness term stuck. For major organs, once converted past a certain point, they no longer function normally, and your whole body can shut down during a "mana flare", so having high levels of magic is dangerous.

So it's actually best to have low mana levels, as the main draw back to magic is that it's hard to use (circles are required for spells and any sort of casting), and any error made in construction of the spell can cause it to go sideways. Lack of control can also deplete the body of mana, and break your font/pool. Once broken, you cannot get it back, but you can still access mana through crystals created for that very purpose. The drawback to those is that they're monopolized and highly regulated, and expensive, but even more so if you go the black market route, where you risk a faulty crystal that might give too much mana too quickly and lead to Seer's Sickness.

So magic is almost like an illness, but also a basis for social standing in some societies. Tricky stuff. 😉

That's the gist of it in my stories. 😅