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."