The only built-in cheat is God Mode.

* Start Abuse with -edit
* Make sure the cursor is in the window
* Press SHIFT and Z at the same time.
* Press TAB to start playing

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Abuse - Demo

Secret levels
The two secrets in the first level are pretty cool, they include a
warp section that lets you jump to any level in the demo.
Here's how you get to them (spoiler):
 

First Secret (get some good weapons)

Kill the two flyers, go down the hole that opens up. Flip the switch
and grab the ammo. Run right and use the teleporter. Then, go right,
stop before you fall down the pit. Jump across the pit to the little
ledge. There's an invisible teleporter there, but the instructions
to press down to teleport will appear. Press down, you're in the
secret area. Grab the ammo and you now have the grenade launcher and
the rocket launcher.

Second Secret (you wanna go where?!)

After you drop down from the bonus weapons, to the left you'll find
a save station. Just to the left of that are some ceiling tiles with
cracks. Blow away the ceiling to reveal a jump-booster. Jump up and
to the left. Run along the upper corridor until a tube opening
appears. Go through it and you'll be in a room with four end-level
thingies (those multi-armed numbers that look like robo-surgeon gone
bad). Each one will take you to a different level...