Sig'sith is a group of conditions that are causing by overusing one's magical capabilities. These conditions have two stages, each one reached by breaking one's limits. Each Sig'sith is caused by the overuse of a specific school of magic and one can be affected by more than one Sig'sith.
Bodies have a limit of magic, both stored and used, and both can be broken with consequences. Sig'sith are the consequences of crossing the limit of the magic used.
Usually (with some exceptions), someone would not be able to cross the limit of their magic, but there are drinks, foods, objects, etc that give the caster a boost. Using these boosts on their own do not cause a Sig'sith, unless used when the caster is close to their limits (something casters learn to identify early on their lifes and trainings).
Another important detail is that a Sig'sith will only occur when the limit is crossed by overusing a special kind of magic, for example casting a strong conjuration spell when already at limit or trying to protect one's body when it is almost a lost cause. The kind of magic used is important to determinate the Sig'sith one will obtain, which is easily identified by the symptoms obtained by the Sig'sith.
Sig'sith are divided into two stages, each one with their symptoms (although some symptoms go to the next stage). Each stage is entered when crossing the limit, something usually described as feeling their heart explode and right afterwards a wave of raw mana crossing all of one's body (there are other symptoms, minors, but these vary depending on the magic used and the person).