When upon taking a small to moderate dose of NyQuil or other OTC sleep aids which contain antihistamine as an active ingredient the ingested pharmaceutical produces undesired side affects such as jitteriness and/or increased heart rate which counter the intended result of sleep.
There is no known cure, treatment is focused on achieving sleep. Common treatments include: large quantities of liquor/cheap beer, more of said pharm, marijuana, drugs...
Man I really wanted to sleep before my flight but i had a NyQuil backfire and I was up all night with a dry ass mouth...
Yo I fuckin hate when NyQuil backfires, fuckin stresses you out the whole next day