The taste of things to come.
Pain is a (normally) unpleasant feeling often caused by intense or damaging stimuli. It's a transitory symptom of an underlying condition or problem in the life form’s body.
It triggers as a burst of pain, then it slowly fades away, causing varying effects depending on the level of pain and its localization. Pain is transferred over the Nervous system layer and is delivered into the brain (so there’s no need for several pain meters for each body part). There is the natural pain resistance every creature has, as a threshold the pain level must cross before causing any effect. Pain resistance is also the factor by which pain is reduced over time.
Pain can manifest in several ways, beside the action message, depending on the level of pain the life form is feeling. Some of those effects are:
Mechanics: Temporarily disabling a body part, which can cause dropping things (in case of pain in arms) or inability to run (in case of pain in legs)
Mechanics: Dizziness, Passing out (heavy stamina damage?)
Mechanics: Vomiting
Visual effects: Sudden red flashes on screen so show damage occurring
Text: RP automated messages like *Twitches*, *Goes Pale*
Sound: Involuntary vocal screaming, moaning, groaning or whimpering.
Animation: Involuntary movement (spasms, twitching), momentarily violent muscle contraction (think of tasers)
Pain can be controlled in several ways. First and most effective way is to fix the issue that is causing pain - take care of the wound, for example.
Drugs such as morphine can also relieve pain momentarily as the needed drug is delivered to the Nervous system via the Circulatory system, without fixing the underlying issue. And lastly there is intervention on the nervous system, affecting Nervous system directly through surgery or other means.