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Antipyretics like paracetamol are used to decrease the body-temperature in fever patients.

But some of them are also used as pain-killers in sprain or other injuries (where there's no fever). In that case they changes the body temperature very little (if any).

Why? Is there any molecular mechanism responsible for this?

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Antipyretics doesn't work by reducing the body temperature but blocking pathways that make it higher.

For paracetamol/acetaminophen the mechanisms implied are not currently completely clear (as reported on Wikipedia or on Katzung's "Basic & Clinical Pharmacology").

For the most common NSAIDs (aspirin, ibuprofen, naproxen, ...) the mechanisms instead are more clear as they block some enzymes that are involved in production on inflammatory molecules (enzymes COX1 and COX2 that are implied in the arachidonic acid pathway as said on this article).

When these inflammatory molecules rise concentration in blood they can get to the hypothalamus so it will set the body temperature at an higher level; when these signals stop coming (inflammation stops or no more molecules are produced because of drug's block) the hypothalamus sets the temperature to the normal range.

More infos about hypothalamic temperature set point.

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  • $\begingroup$ Ibuprofen acts as antipyretic? $\endgroup$
    – user25568
    Commented Sep 8, 2016 at 11:50
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    $\begingroup$ Of course yes (but paracetamol is preferred if only anipyretic effect is needed). For example ibuprofen in Italy is the only official alternative to pracetamol as antipyretic in paediatric patients. $\endgroup$
    – DavideN
    Commented Sep 8, 2016 at 12:40

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