While the Flash is easily one of the most powerful heroes in the DC Universe, his abilities do have a number of side effects that could prove detrimental to the Scarlet Speedster, including one horrifying weakness that was actually revealed by Marvel Comics.
Wally West aka the Flash is able to pull off astounding feats of heroism because of his connection to a cosmic power grid known as the Speed Force. By channeling the Speed Force, the Flash can sprint through the time stream, heal incredibly fast, and even separate his molecules to phase through objects at will. There are a few catches to these powers, however, including his heightened susceptibility to drowning as every tiny bit of water he could accidentally breathe in while swimming would fill his lungs in seconds due to his relative speed/breathing rate. While that side effect to super speed does pose a significant danger–especially if Flash ever wanted to help out Aquaman on a mission– Marvel pointed out another cost to his powers that would assuredly kill the Flash in a matter of months.
In Uncanny X-Men #527 by Matt Fraction, Whilce Portacio, and Leonard Kirk, Marvel Comics introduces a mutant named Gabriel Cohuelo aka Velocidad. Like the Flash, Velocidad is a speedster whose body is greatly impacted every time he uses his powers. Velocidad’s powers allow him to slow down time while he remains unaffected, effectively giving him super speed, though his body ages at a relativistic rate during this process. This means that he could age months in a matter of moments if he strenuously uses his powers.
Based on the fact that the Flash’s metabolism and body functions speed up because of his powers, he should logically also suffer the same fate as Velocidad. In JLA #66 by Joe Kelly and Doug Mahnke, the Flash revealed that he has over three hundred sleep cycles per day. Based on his other bodily functions that have been impacted by his powers, the fact that he goes through over three hundred sleep cycles per day means his body should age at a relativistic rate, meaning he should be aging roughly one year per day.Unlike Velocidad who only suffers this grim side effect when he uses his powers, the Flash’s body is constantly using his powers one way or another, so his life should be constantly draining with every breath he takes which would make his situation way more dreadful.
While DC has never hinted that the Flash’s powers make him age at a rapid rate, Marvel Comics took the rules set by Flash’s biology and applied it to an original character within its universe and didn’t pull any punches in terms of depicting the horrifying cost of his powers.