Quote Originally Posted by Lyzuu
What I don't get is how a pain reliever works. Can anyone explain this to me (backaches, stomaches, and any aches)?
Loads of stuff works differently, via different mechanisms.

1) General pain relief - paracetamol: mechanism is debatable. In the past, it's thought to inhibit the COX1/2 feedback, leading to reduced pain, yet minimal anti-inflammatory effect. It's also thought to be selective for targets in the brain, making it ideal for headaches, but rather useless for inflammatory pain. It has a very good side effect profile, making it a very safe drug when taken at therapeutic doses, but overdosing can have severe consequences

2) Inflammatory pain relief - NSAIDS: These work by directly inhibiting the COX-1/2 enzymes, in turn inhibiting inflammatory mediators and reducing inflammation. NSAIDS that block both COX-1 and COX-2 enzymes are subject to gastro-intestinal side effects, primarily due to COX-1 inhibition. COX-2 selective inhibiting NSAIDS have a much lower affinity for COX-1 receptors, and so incidence of GI side effects through use are much lower. However, highly selective COX-2 inhibitors will cause increased levels of pro-clotting factors, increasing the risk of stroke.

3) Opioid pain relievers - These pain killers work by binding to certain (and sometimes multiple) opioid receptors. According to a paper "Opioids, to varying degrees, may antagonize Nmethyl-
D-aspartate (NMDA) receptors, activating the
descending serotonin and noradrenaline pain pathways
from the brain stem. Stimulation of these same
NMDA receptors may result in neuropathic pain and
the development of tolerance"

4) Stomach aches (indigestion, heart burn, reflux): These types of pain are caused by the excess of stomach acid and/or poor closing of your sphincter, causing gastric juices to reflux into your esophagus, which has no protective lining. Antacids are simply basic chemicals designed to neutralise stomach acid. More potent drugs include Proton Pump Inhibitors and H2-Antagonists. These aim to reduce your production of stomach acid.

There's a brief outline of the differences. By no means is this all there is to know. If you want to know more, you can try wiki, the references they use, and other medical search engines like Pubmed. (in increasing order of accuracy).