Caution

Super long text blocks below. They are also disorganized and unedited

Conversely, magic is helpful to your body (goes through your blood stream and increases the efficiency of blood circulation, fights infections and such because it is directed by your will and your body really wants to survive, can form membranes in place of damaged tissue, many things) and while not necessarily malicious on it's own, your body becomes reliant on it because why continue expending resources on processes that are now happening "on their own"?

Magic will do whatever you want it to do, you need only think it. However it will not continue doing something if nothing is causing it to. This has been likened to a container of gass. If you remove the balloon, the air inside will not stay fixed in the same position. And if you stop willing magic to do anything it will stop doing that thing, as there is always thought happening. Mending another's wounds can be a horrific thing to do to someone and while it might be hypothetically possible to use magic to transform existing matter into flesh to heal someone, the magic is practically impossible to remove from the wound as you would have to think about it without thinking about it.

Magic makes your thoughts better at commanding magic, but either thoughts can draw in magic without having any, or all living things have some magic that can not leave (until they die or otherwise somehow completely stop thinking? Unknown/writer's decision)

It is horrifying that a fundamental and inescapable aspect of existing makes it dangerous to think. Thankfully it will effect you less if you avoid using it.

I just thought of a way gods or spirits could work. Thoughts can rub of on magic, and over an unknown but very long time this has, and surely still is, forming magic that is capable of thought. Spiritsand gods, like magic and all other things, can never be destroyed. They can only be changed. Thus immortality. Of course will can fade from magic over time so gods and spirits need to be around thoughts, or preferably thought about, or even better worshiped. Due to this who and what they are can be affected and even controlled by how they are perceived.

Particularly Complex thought

I am not sure whether the difference between gods and spirits lies in terminology or power, but the difference between spirits or gods and ghosts I have come up with. Ghosts were alive as mortals. A ghost is what happens when someone is fully ship of theseus'ed. Once all living tissue is replaced by magic (this can be due to dying, none of the tissue is living) your entire body must constantly be held together by your mind. This makes doing anything rather difficult, so ghosts get tired out from doing things like:

Making noise

Physically existing, even partially

Particularly complex thought

Being visible

And so on. Hypothetically a ghost could become a spirit/god, but this would take a lot of people working very hard (mentally) and/or literal divine intervention.

Since magic is controlled by thought, being more magic makes you more able to control magic, a magic user is more vulnerable to magic the more powerful they are. Kinda. The more of you is magic, the more magic you can control, but the more magic there is, the harder it is to control. So while someone with more magic would be more effected by the magic in them being altered or moved they also have more to alter or move so it is harder to do so. The more firmly planted in it is the more damage is done if it's removed, but it is easier to remove if it is less attached. Of course it is very difficult to remove magic as you would need to focus on the person or object you are removing it from and the magic without just directing the magic into what you are thinking about

Some of my favorite magic side effects:

-Nosebleeds. Never gets old.

-Coughing up blood. The good ol’ “cough into your hand and pull it back to see blood” also never gets old.

-Headaches. You keep fighting as your head pounds, desperately telling you to take a break. At first they fade within minutes when you stop using magic, but overtime, they become chronic.

-Fatigue. After a big battle, you stand triumphant, and then just fall asleep on the spot.

-In a similar vein, overuse causing you to straight up faint rather than just fall asleep. Darkness begins to overtake your vision in the middle of battle, unconsciousness abruptly looming over you.

-Any of the side effects happening to another person. Maybe two close characters are connected, and whatever side effects character A would normally endure are transferred to character B. When A uses a blast of magic B screams loudly because holy shit that hurt.

-Magic gradually deteriorating your mind. Using it too much eventually caused hallucinations and an inability to retain memories, or even larger scale memory loss.

Feel free to add more, I’m looking for some to steal