Orgasms, exercise and other stimuli make your blood vessels shrink (vasoconstriction). From the link above: “During arousal, your sympathetic nervous system — the one that activates your fight-or-flight response — kicks in. Just as when you exercise, your adrenaline levels go up and your blood vessels constrict. Less blood flow to your nose means less inflammation, so your nose opens up and you can breathe more easily.”

THC can cause vasodilatation, which is the opposite (blood vessels expand) so it makes sense they’d reduce effects or cancel each other out. Fun fact from the same article, it’s also what causes the bloodshot eyes associated with smoking up.