Eli5: How is paracetamol used for both fever and as a pain relief tablet?
The mechanism of action of paracetamol as a pain relief drug isn’t perfectly understood as it appears to behave differently in a lab and in an actual person. But the reason it can do both of these things is because it can act on 2 different parts of the nervous system.
It’s believed to reduce pain by interfering with an enzyme called cyclooxygenase. This enzyme *usually* produces chemicals called prostaglandins which can cause inflammation. So inhibiting this reduces the amount of inflammation and therefore pain. Another theory for how they work is that something your liver breaks paracetamol down into may act on cannabinoid receptors, which are responsible for pain control among other things.
As for how it controls fever, it may be able to do so both directly and indirectly. If there is systemic inflammation (this is inflammation that is widespread throughout your body) then reducing the amount of inflammation through the first above process can bring your temperature down, as heat is a component of inflammation. However, as I mentioned, we’re not entirely sure how paracetamol works exactly and it’s effect on inflammation is debated.
But it has another way to reduce fever, and that’s by acting directly on the hypothalamus – this is the control centre for homeostasis in the body. The hypothalamus tells you when you’re hungry, thirsty, cold or hot etc and it can both send signals to your organs to make them behave in a way to fix these (for example telling your kidneys to hold on to water if you’re dehydrated) and it can send signals that we interpret as motivations to change them, such as telling us we’re thirsty so we go and get a drink. Normally, your body temperature should be controlled at around 37 degrees C. In a fever, the hypothalamus basically says to the body “No, you need to make us 39 degrees” so your body starts doing things to conserve heat or produce more, for example shivering or constricting blood vessels near the skin. So you get hotter. Paracetamol has the opposite effect, it convinces the hypothalamus to bring the temperature back down again so you stop shivering, your blood vessels dilate etc and then you lose heat until you’re at the normal temperature again.
As an analgesic, it has properties that numb nerve endings. This provides the pain relief as the nerves are no longer firing to send the OW! signal to the brain, but also, it stops all signals.
Therefore the heat that we feel from a fever is a reaction that bad is happening in the body, so send lots of blood in and the heat with kill off the infection.
If the nerves that detect the bad happening are no longer sending that panic signal, then the nervous system thinks that the infection is dealt with and readjusts the system to remove that excess blood. Blood levels return to normal, so heat felt (fever) returns to normal too.