There are several different classifications of medications that are often used for pain. I'll try to provide some information on these medications here.
The first of these pain medications is acetaminophen (Tylenol). The exact mechanism of action is unknown but it is thought to work at cyclooxygenase-3 in the brain. When taken in normal levels, not to exceed 4 grams per day (I hear they are thinking of decreasing this to 3 grams per day) for those with an intact liver, it is very safe and can be very effective.
It is effective for a variety of pains from musculoskeletal to headaches.
This medication can be extremely toxic in levels over 4 grams per day. The thing to keep in mind about tylenol is that it is also present in a number of other medications such as Lortab, Vicodin, Percocet, Norco, Tylenol #3, Nyquil, and many over the counter pain, cough, and cold medications. The key is to account for ALL of the acetaminophen you are taking.
NSAIDs (Non-steroidal anti-inflammatory)
There are several classes of NSAIDs with different potencies and different tolerabilities. However, they all act at the cyclooxygenase 1 &/or 2 enzyme effectively decreasing the production of prostaglandins.
Prostaglandins are those pesky chemicals that our bodies produce via the Arachadonic Acid pathway that are responsible for many of the effects of acute inflammation. Blocking the COX-1 &/or COX-2 enzymes decreases these prostaglandins which, in effect, decreases our pain.
However, prostaglandins also serve a variety of good functions that we do want. Taking NSAIDs also decreases the good prostaglandins. Therefore, these medications should be used sparingly and with caution. For example, NSAIDs can affect the normal function of platelets and should NOT be used in patients at high risk for a heart attack.
These medications work by interacting with the mu opioid receptor. These medications activate this receptor which suppresses pain - they work at other receptors and have some other effects such as respiratory depression and reduced gastric motility (causes constipation) as well.