As we know, magic is simply the power to manipulate energy, which is used to perform supernatural acts, ranging from the basic mesmer to an extended lifespan.
Each fairy family possesses some kind of magic.
But the question is, what happens when you use too much of it?
According to the Law of Conservation of Energy, energy can neither be created nor be destroyed. It can just be converted from one form to another.
And though there are many types of energy, it is divided into two main types:
1) Kinetic Energy (energy present in anything in motion) and
2) Potential Energy (energy stored within a stationary object, due to the object's position, arrangement or state).
Alright, enough of Physics lessons.
Time to add a pinch of magic.
Now say, if I give you a rubber band. In its original state and shape, it possesses potential energy. Now, stretch it. In that moment, when you have given motion to the rubber band, it has converted its potential energy to kinetic energy. Now, again if I ask you to continuously keep stretching and relaxing it, it will break (just try not to hurt your muscles though, they follow the same mechanism too).
Why does it break?
Because you are constantly converting, i.e., manipulating its energy, right down to its subatomic level, where the rubber molecules cannot bear converting kinetic to potential and potential to kinetic energy back and forth, again and again, they simply dissipate at the point (so sound energy) where maximum transfer of energy has taken place, causing the rubber band to break (is this even true? I am not much of a Physicist).
There you are! So, if it has been correctly hypothesised, then you can interpret that if a fairy uses magic all the time, especially on a particular object/subject, either they will be harmed (because the magic is flowing from within them, converting the energy inside them to another form), or the object/subject will (as with the poor rubber band).
To apply it in Fowl Universe, we have very clear-cut examples. If too much of healing is performed on an individual and magic overloads their system, combined with negative traits of the individual (such as feelings of guilt), it can cause severe diseases or disorders (like Atlantis Complex).
If too much mesmer is performed on an individual, it can numb their thinking and also caused misshapen pupils.
The key to using magic effectively is using it in balance, no matter what the use.
I hope I could explain my views on the topic, and would definitely like to see everyone's views on this.
Special thanks to @Succulents-and-Sage who originally came up with the topic.