Why is acetaminophen not very effective in treating arthritis?

Answer 1

First off, acetaminophen is great for Arthritis. This question stem is wrong, and I'll explain why.

Mediators of pain and inflammation are created in your body by an enzyme named cyclooxygenase (COX). This enzyme is therefore a target of drugs such as aspirin, ibuprofen, and perhaps acetaminophen amongst others. Aspirin and ibuprofen directly interact with this enzyme, binding it and thereby inhibiting it. Acetaminophen's mechanism is more or less unknown, but is thought to inhibit COX through another mechanism: scavenging COX's substrates. COX needs free radicals to function. These radicals are a reactant in the chemical reaction it catalyzes. Without radicals, COX cannot proceed.

When our immune system is fighting an a bug, our white blood cells release radicals to kill the infection. Sometimes our white blood cells perceive ourself as foreign, and mount a similar attack. Inflammation increases free radicals. If we take acetaminophen during an inflammatory process, the drug will be overwhelmed by all of the radicals (because it binds them, remember?) If the acetaminophen is exhausted, this means COX function will be uninterrupted. This makes acetaminophen less effective in managing the pain and inflammation secondary to an inflammatory process.

This being said, osteoarthritis (from degenerative changes/loss of cartilage) is NOT an inflammatory process (we don't know why steroid injections work), and so acetaminophen IS a great treatment for it. This is especially true because osteoarthritis is a long term problem that requires long term treatment, and ibuprofen or aspirin or naproxen etc. would likely cause peptic ulcers if taken that long, and so acetaminophen is a first line choice. This question stem is misleading and incorrect. Celecoxib/celebrex is an NSAID selective for cox 2 enzyme, easier on the stomach and another option for long term pain control of osteoarthritis.

Acetaminophen would NOT be a great choice for arthritis from an inflammatory etiology, such as rheumatoid, or connective tissue diseases like lupus, for the reasons before explained. Acetaminophen is also not the best choice for any pain caused by tissue injury (back sprain, torn ligament etc) because of reasons explained before (excess free radical release from WBC's cleaning up the mess).

There is more to acetaminophen's mechanism than we understand. It apparently doesn't inhibit COX all that much, because COX enzyme is critical in making prostaglandins that dilate the kidney's arterioles. NSAIDS are a big no no in someone who is old with already low kidney function or in someone with bad kidney disease like Diabetes. However, acetaminophen is great for pain management in people with kidney malefaction because, although we believe it inhibits COX via scavenging radicals, it apparently doesn't have a big enough effect on COX to inhibit prostaglandins and cause a decrease in GFR.

Answer 2

Dr. Peter Gott, who writes a syndicated column on medicine called "Ask Dr. Gott" is on record as calling Tylenol (acetiminophen) "a rotten pain reliever for adults". Countless adults agree with him. Among OTC NSAIDS, Ibuprofen (Advil), Aspirin (if you tolerate it well) and Naproxen sodium (Alleve) are all more effective.