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1.
A residual drug or alcohol phenomonon in which, during a period after a heavy binge, only a small amount of the recently-abused substance is needed to re-activate your buzz. So named after what happens in the shower when you "rinse and repeat": only a tiny bit of shampoo is needed the second time around to achieve a full sudsy lather.
"Man, I've only had one bloody mary and I feel buzzed!"

"Shampoo effect, dude. You were blasted last night."
by Soave May 23, 2005
 
2.
When because of recent use, a little goes a long way. Usually referred to when re-gaining an alcohol buzz. (If you shampoo your hair and ever "repeat", you know how just a tiny bit of shampoo will cause instant lather? that's the shampoo effect)
I had so much to drink last night, all it took was one beer this morning to get me buzzed again, gotta love the shampoo effect!
by Angie C September 14, 2005
 
3.
what happens when you drink alcohol after previously drinking alot the night before and you get buzzed faster, similiar to when you shampoo your hair and if you wash it twice in a row the second time it gets alot foamier faster the second time
i need to drink a little slower right now i already have the shampoo effect going on after last nights binger
by free mustache rides November 06, 2010
 
4.
The phenomenon of when it takes just a little bit of booze to get drunk the day after a night of many alcoholic beverages. The same way as when a bottle of shampoo is almost empty and it just takes a little bit of water to make more shampoo.
"Man im feeling drunk after these 2 beers for some reason. It must be the shampoo effect going on from all those shots I did last night."
by Gun5linger March 24, 2012
 
5.
Similar to how adding water to a nearly empty shampoo bottle gives you "more shampoo", giving a small amount of alcohol to someone who was very drunk the night before, returns them to a drunk state.
Bro 1: "Damn, why is Alex so drunk right now? He's only had two drinks."

Bro 2: "It's the Shampoo Effect, he was plastered last night."
by icechris February 06, 2012
 
6.
The effect when you drink alcohol the day after getting extremely drunk and it only takes you a quarter of the amount of alcohol to get you equally as drunk as the night before.
The shampoo effect really got Brittney tonight, she only had one shot and now she is peeing in the refrigerator!!
by slevin20 January 01, 2011
 
7.
(noun) The effect of having that first drink the next day, after a long night of drinking, and it gives you a good buzz right away. Much like how quickly the shampoo lathers as you apply it the second time.
Whoa, dude, I'm gettin' a heavy shampoo effect from this morning's bloody mary.
by Edward Cafeto April 11, 2006