A stun grenade is a thrownweapon that appears in Fallout: New Vegas, Fallout Tactics and Van Buren. Stun grenades, also known as flash grenades or flashbangs, are non-lethal grenades used to disorient and distract targets by producing a blinding flash of light and loud sound, without causing permanent damage. The bright light over-stimulates the nerves in the eye, causing temporary blindness, and the loud sound causes temporary hearing loss and loss of balance from disruption to the fluid in the ear.
A less-lethal alternative to a traditional grenade. Flash bangs deal very little physical damage, but the blinding light and deafening bang it creates deal significant Fatigue damage.
This grenade cut from Fallout: New Vegas was meant to inflict fatigue damage to targets, allowing for a less-lethal explosive weapon. Typical of stun grenades, a bright light accompanies the detonation, which can momentarily blind or blur vision.
The following is based on Van Buren design documents and is not canon.
Flash grenades, by themselves, do very little harm. They instead use a magnesium core to release a blinding flash upon detonation, stunning opponents caught in the blast radius.
End of information based on Van Buren design documents
End of information based on information from Fallout Tactics