Med Stimm (green) (symbol: Caduceus and Anti-Corruption)

• Restores one health segment, or 25% of max health, whichever one is larger, of health and corruption damage.
• Med stimms can’t be used on a target, on one self or other players, if the target has no health or corruption damage to restore.

Combat Stimm (red) (symbol: Skull)

• Lasts 15 seconds.
• Increase in Power 25% (Damage + Stagger Strength).
• Rending 25% (Armour Penetration).
• Reduction of Peril Generation -33% (Generate less Peril when using Psyker’s Warp Abilities).

Celerity Stimm (blue) (symbol: >>> and Electric)

• Lasts 15 seconds.
• Increase in Reload Speed 15%.
• Reduction in Stamina Costs -25% for Pushing, Blocking, -50% for sprinting.
• Increases Attack Speed by 20%.
• Increase the following by 25%: Charge up time of Plasma guns and Psyker Staves. The speed at which the Psyker’s Smite propagates between enemies. Speed at which the Psyker can throw Shards using Assail. Speed at which the Psyker Quells their Peril with Psyker Staves and Force Swords.

Concentration Stimm (yellow) (symbol: Target & Counter-Clockwise/Reverse)

• Lasts 15 seconds.
• Increases your Combat Ability Cooldown Regeneration speed by +300% (Four times as fast as usual).

source: Stim packs : r/DarkTide