Concussions Get You High

In real life, concussions have a number of unpleasant symptoms. These include, but are not limited to headache, nausea, and dizziness. They also involve minor brain damage in addition to those. Fortunately for our protagonist, this is fiction. Here, if one gets a Tap on the Head or a particularly violent Dope Slap, one need only worry about sporting a dopey grin and falling over, or, at worst, experiencing a Non Sequitur Thud. In fact, the effects of Slapstick concussion look alarmingly like the effects of euphoric drugs.

(Don't worry about after-effects, either - any Circling Birdies can be dismissed with a quick shake of the head, and a Cranial Eruption can often be pushed straight back in. And multiple head injuries will never make the problem worse, ever.)

Naturally, depending on the work, this falls under either Artistic License, Acceptable Breaks From Reality, or, if the writers are truly dense, Critical Research Failure. Surprisingly though, there can be a grain of Truth in Television in it - someone acting mildly drunk after taking a blow to the head is a good indicator of concussion, and they should probably see a doctor.

Related to Hard Head, Tap on the Head.

Examples of Concussions Get You High include:

Advertising

A Sega Game Gear ad involves someone hitting his head with a squirrel to see colour in their Game Boy: http://www.youtube.com/watch?v=nBLr4wbiR6E

Film

The Three Stooges would often use this, by having whoever was hit on the head take on a silly facial expression and slump over to the sound of chirping birds.
In the Chinese film Kung Fu Genius, one of the Big Bad's lackeys takes too many hits to the head in one fight, and instantly becomes a child-like Cloudcuckoolander for the rest of the movie.

Live Action TV

In the Boy Meets World Episode "Easy Street" Erik gives himself a concussion trying to escape from a car that was buried in the snow. Upon recovering Erik is loopy, cheerful and generally appears pleasantly intoxicated.
On The Office, Jerkass Dwight starts acting uncharacteristically nice after crashing his car. The others eventually realize that he has a concussion and take him to the hospital. During this bout of identity amnesia Dwight also shows absentmindedness and compulsive behavior not unlike fictional portrayals of being high on marijuana.
In the Young Blades episode "Rub-a-Dub Sub," a character gets an anchor dropped on his head. The episode tag features him in a dungeon, delirious and spouting random nonsense.

Western Animation

In the Disney short Clock Cleaners, Goofy gets hit on the head while on a tall building, and wanders onto a construction site afterward, completely entranced.
In the Chip and Dale short "Working For Peanuts", Donald Duck chases the two chipmunks until slamming his head into a wall after they escape through a hole. He then sits there with swirly eyes and a toothy grin on his face; later on in the short the elephant he was taking care of finds him still like this.
A Sealab 2021 episode had the crew trapped in a malfunctioning storage closet. Beard Guy comes in to fix the problem, but Captain Murphy beats his head in. Upon briefly regaining consciousness, he displays this trope.

Beard Guy: I got my think a concussion. Now must remember not fix stuff. Gonna lay down back, cuz our concussion have us sleepy.
Debbie: Uh, should Beard Guy be asleep with a concussion?
Quinn: No, absolutely not.

An episode of Ren and Stimpy features Stimpy excusing himself because "It's time for my appointment". He walks over to a door in the wall, inserts a quarter, and the door lifts to reveal a horse. The horse kicks him in the head, sending him flying. His reaction to this is almost orgasmic.
A noticeable aversion from Ozzy and Drix. When Hector takes a nasty spill off his bike, he displays actual symptoms of a concussion. Noticeably, intense dizziness, and confusion.