They come in three types:
When taken, Cooper's actions are greatly reduced in speed. This includes running, attacking and using weapons. Taking another one when one is active will not make him even slower or even prolong use. Also, taking a Turbo Cooper! after taking a Slow-Mo Cooper will instantly stop the effects.
When taken, all controls are reversed, meaning running and attacking will go the opposite direction. This does not apply to picking up weapons. Taking one when one is active will not reverse the controls again (thus fixing it) or even prolong use
This will decrease Coopers Heart Points into 1, leaving him very vulnerable. Taking one when one is active will not prolong the use.