Most mages in the Marvel World drew on other dimensions or powerful magical entities when casting spells. The power of dark magic, for instance, came from the Underworld God Chthon, while the spells of the Kamar-Taj tradition drew their power from the trinity known as the Vishanti. The Ancient One had extended her life by drawing magic from the Dark Dimension, bypassing Dormammu, its ruler.
The same was true of supernatural abilities, not just magic. Take the Phoenix Force of Jean Grey from the X-Men, for example. Cyclops didn't fire red concussive beams from his eyes; rather, his eyes could open a portal to a dimension filled with high-density energy. The difference in energy density between the two dimensions created a shock wave as they moved toward equilibrium.
Then there was the Sentry, one of Marvel's four knockoff Supermen. He had opened a gateway to a single universe with a vial of serum, gaining the power of a million exploding suns.
But unlike those who relied on random mutations with practically zero odds, magic was a learnable ability, and casting spells came at a price. Organs, mental strength, souls, bloodlines—anything you possessed could be paid as a price.
The reason Doctor Strange was called Kidney Deficiency the Sorcerer was that his excessive use of magic had left his body in tatters. He was already completely wrecked.
He could sleep for only three hours a day. Otherwise, his damaged soul would come into contact with the mad realm within the Dream Dimension while he slept. The terror of encountering it would drive him insane, and he even coughed up fragments of his soul every day.
Doctor Strange's stomach was nearly destroyed, too, leaving a hole the size of a mouse that neither magic nor technology could repair. He survived on porridge