Eli5: Why is caffeine often recommended for headache relief?
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Caffeine is a vasonstrictor (narrows vessels) so it clamps down on vessels in head that can cause headpain, ie headache. The opposite of a vasoconstrictor is a vasodilator (opens up vessels) such nitroglycerin which causes headaches. Another famous vasodilator, sildenafil, opens up some other important vessels (and also known to cause headaches)
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