What is flashs weakness?

Answer 1

The Flash has no specific weakness like Superman's Kryptonite.

The Flashes (of which there have been four) are still technically human so bullets/weapons andundernourishment/fatigue could effect them the same as they would any human. They have ways of making these problems unlikely, however, such as keeping food storages in their suits and evading bullets and other weaponry using their own speed.

Some villains may also use Flash's speed against him (slippery ice, speed-activated bombs, etc.) or even negate his powers, making him completely human.

Also, The Flash has been known to suffer horrible consequences for going too fast, such as being sent to another timeframe/universe or being sucked into the Speed Force (as occurred in the Crisis On Infinite Earths). However, the speed which is thought of as "too fast" or uncontrollable can vary from comic to comic.

Answer 2

Flash's Weakness Is Gravity

(Justice League s. 2 ep. starcrossed part. 3)