How does ibuprofen work?
Ibuprofen is classed as a non-steroidal anti-inflammatory drug or NSAID. It works by acting on a group of compounds called prostaglandins.
Prostaglandins are sometimes called local hormones because they act close to where they are produced rather than all over the body. They have a remarkably wide range of effects. One of their actions is to cause inflammation.
When a part of the body is injured, protective mechanisms go into action. White blood cells accumulate at the site of the injury, and this causes swelling, heat, redness, loss of function, fever and pain - together called inflammation.
All these effects are potentially beneficial. Swelling can help to immobilise injured joints, heat and increased blood flow promote healing, and pain alerts the injured person that there is a problem (so that he or she will quickly remove their hand from a hot stove, for example, to prevent further injury). However, they can often be too much of a good thing - once we know we are injured, the pain no longer has a function and we wish to be rid if it; often the inflammatory response is too powerful and can do more harm than good.
Prostaglandins are involved in bringing about the inflammatory response. Ibuprofen’s action as a painkiller and antipyretic (fever-reducing) compound is due to its ability to inhibit the synthesis of prostaglandins. It does this by interfering with the action of an enzyme called cyclo-oxygenase which catalyses the conversion of a compound called arachidonic acid into prostaglandins, (see Figure 4). Aspirin and other NSAIDs work in a similar way.
Figure 4: Prostaglandin synthesis
At first sight you may think that the structures of arachidonic acid and prostaglandins are very different but imagine a bond forming between carbon 8 and carbon 12 in arachidonic acid to form the five-membered ring of the prostaglandin and you should see the similarity. (The carbon atoms have been numbered from the -COOH end of the molecule.)
Side effects of NSAIDs
In common with other NSAIDs ibuprofen has some unwanted effects (side effects). This is because prostaglandins have many other roles in the body as well as their control of inflammation. Inhibiting prostaglandin synthesis inevitably affects these too. Two of these roles are that they help protect the lining of the gut and that they are important for blood clotting to work properly. So taking NSAIDs can cause irritation of the gut and reduced blood clotting, which makes bleeding more likely. However, taking the correct dose ensures that the positive effects (a reduction in pain, swelling and inflammation) outweigh the undesirable ones (possible bleeding in the gut). Ibuprofen causes less stomach irritation than aspirin.
The difference in dose between the start of the beneficial effect and the onset of side effects is called the therapeutic (curative) window.