All cheats can be activated once per level:

 HEALME          - Complete healing
 INEEDABIGGUN    - Gatt powerup
 TORNADOAWAY     - Twister powerup
 HOLOKURTISFUN   - Dummy powerup

 ---

 HexCheat:

 Load the file mdk95.exe into the hex editor and change the following
 offsets:

 offset       content    change to

 --------------------------------------
 76487        29         90
 76488        C2         90
 --------------------------------------

 Now you are invulnerable.

Cheats:

 Each cheat can be used only once per level.

 Codes for ALL of the Patches:

 Type all of these codes during game

 ineedabiggun    = Super Chain Gun powerup once per level
 holokurtisfun   = Dummy powerup once per level
 nastyshotthanks = Homing sniper grenade once per level
 tornadoaway     = Twister powerup once per level
 486ok           = Allow 486 machines to run

 For Un-patched final North American executable:

 makemefull      = Health once per level
 masterblaster   = Gatt powerup once per level
 twistandshout   = Twister powerup once per level
 biggrenade      = Homing sniper grenade once per level
 486willbeslow   = Allow 486 machines to run

 For European Version (Should work for No Blood German versions as
 well):

 healme          = Health once per level
 ineedabiggun    = Gatt powerup once per level
 tornadoaway     = Twister powerup once per level
 holokurtisfun   = Dummy powerup once per level
 486okbyme       = Allow 486 machines to run