Spell sickness is a term used to describe the aftereffects of performing magic in the Briarverse.
Symptoms range in scale and severity and seem to be proportionate to the intensity of the magic used. A group spell, such as a grounding hex, might result in only a headache or fatigue, but casters have also experienced flu-like symptoms, feelings of nausea, or no symptoms at all. However, the Living Spirit spell had severe chilling aftereffects on the Mountaineers who dream-walked and were found by the spirits of former dream walkers during the Search for Magiq initative, and the toll of keeping the Communion Magics circuit open had literal crushing effects on Eaves, who blacked out and experienced fractured ribs and broken fingers when a circuit that had been open for nearly two weeks finally closed.
Spell sickness is mainly treated with rest. Caffeine is also known to help abate some symptoms of fatigue.