Berserk

The berserk pack is a powerup that looks like a black medikit. Picking one up will boost your health to 100% and fill your vision with a red haze of drug-crazed fury. While you are under the influence of a berserk pack, your fist attack is powerful enough to smash weaker enemies to a pulp, or kill a demon with a single punch.

When you collect a berserk pack, your current weapon will switch to the fist (you're so enraged you just want to smash the bad guys' teeth down their throats).

Berserk packs increase the strength of your punch tenfold until the end of the level (not just until the red haze fades from your view).

In original Doom, if you have a chainsaw, you cannot switch to fists unless you have berserk.