What is the treatment for nitrogen narcosis?

Answer 1

don't Scuba dive in deep water. Stay above 80 ft. of depth.
Helium may be used as a substitute for nitrogen to dilute oxygen for deep water diving. It is colorless, odorless, tasteless, and chemically inert. However, it is more expensive than nitrogen and drains body heat from a diver.

Answer 2

The effects of nitrogen narcosis are totally reversed as the gas pressure decreases. They are typically gone by the time the diver returns to a water depth of 60 ft.
There is no real "treatment" as such, but the symptoms usually alleviate rapidly by reducing your depth.

Addition

The above is correct, except that ascending to a shallower depth is in fact the treatment and narcosis will always be reduced when a diver ascends.