Turn on, tune in, toad out.

"I'd like to remind all my readers, especially you impressionable young people, that if you must lick a toad, make sure it's wearing a condom. Thank you."

— Dave Barry

Licking the backs of toads to get high.

Based on an urban legend dating as far back as The Seventies, groups of hippies and teenagers were alleged to have been licking specific species of toads which secrete psychoactive compounds through their skin that can cause hallucinations.

While there are toads that do secrete such chemicals, licking it off a toad itself is very dangerous (they are toxins to deter predators after all, coupled with an easy case of salmonella) and will cause some serious poisoning for anyone considering a pursuit to get high in such a fashion in Real Life (or in layman's terms, Don't Try This at Home).

In fiction, the psychoactive toad is commonly used as a G-Rated Drug.

Examples

Comic Books

One X-Men spinoff series featured Toad Boy, a young mutant who secreted a powerful narcotic.

Film

There is an Australian documentary about cane toads, which covers using toads as hallucinogens.
Hippy: Don Juan the Yaqui brujo says that when you take peyote, you become as one with the spirit of the cactus ... yer toad's the same.

Literature

In one of the Sword of Truth novels, hallucinogenic toads are used as part of a tribal ritual, but they're not actually licked as the hallucinogens in question are transmitted through skin contact.
A character in Big Trouble has an encounter with a toad of this nature, and spends the rest of the book firmly believing that a politician's wife is trying to eat his soul.
Carl Hiaasen's eccentric governor-turned-hermit Clinton "Skink" Tyree frequently licks toads.
The Anarchist Cookbook has a recipe to make dried toad skin powder for smoking. It's not good for your toad karma.

Live-Action TV

One episode of The X-Files (Quagmire) had a bunch of slackers licking toads to get high.
Snorkel Guy: Dude, what is wrong with you? You made me drop my toad!
A hallucinogenic toad turned out to be a major plot point in an episode of Jonathan Creek. The toad wasn't native to England and had been imported by a local hippie colony for this very purpose. The presence of the toad, allegedly in the prime suspect's house, was what led to Jonathan solving the case he was working on.
An episode of Skins (the American version), features the kids trying to get high from licking a toad.
When the gang from Mystery Science Theater 3000 riffed Jack Frost, there was a talking mushroom man; the characters wondered what a talking mushroom would eat to get high, and the only answer anyone can come up with was, "I think they lick toads."
The solution to the Eleventh Hour episode "Cardiac" involves kids attempting to get high off frogs.
One the escapees from Hell in Reaper was a stoner whose preferred method of getting high was licking toads. Fittingly, his demonic power was having a prehensile, Overly Long Tongue.

Music

The M83 track "Raconte-Moi Une Histoire"*
"Tell Me a Story"
is nothing more than an instrumental with a voice-over of a little girl telling of the wonders of touching a special little frog in the rainforest.

Tabletop Games

In Planescape, there's a bit of flavor text that quotes a Sensate's opinion on licking a Slaadi. He says that it is a horrible mistake... that everyone should make once in his life.

Webcomics

One strip from Head Trip claims that James Cameron came up with the inspiration for Avatar after licking a toad.

Web Original

From Things I Am Not Allowed to Do at Hogwarts: "I will not lick Trevor."

Western Animation

Homer in The Simpsons episode "Missionary: Impossible" is depicted at one point lying on a hammock and picking up toads at random and licking them to get high while stranded on a South Pacific island.
Bart: Dad, are you licking toads?
Homer: I'm not not licking toads!
- In another episode, Homer resorts to licking the toads in a pet shop in an attempt to intoxicate himself after being kicked out of Moe's Tavern.
- In one more, Lenny licks a toad after a drug test held at the plant.
On Family Guy, "toad" becomes such a popular fad at James Woods High School, after a Colombian drug cartel's plane transporting the toads crashes near Quahog, that Peter winds up going undercover at the school as "Lando Griffin" to get the students to stop.
- There was even a hilarious anti-drug PSA directly spoofing the classic Tootsie Pop commercial with Mr. Owl.
Kid: Mr. Toad, how many licks of you does it take to get to the center of a Rhode Island State Prison?
[Jail Door Closes]
Voiceover: Just one.
In Avatar: The Last Airbender, toad-licking is used as a fever cure.
An episode of Drawn Together saw Xander, Toot, and Wooldor discover that Ling Ling excretes a hallucinogen whenever he becomes disappointed. After directly referencing the similarity to toads, they proceed to repeatedly disappoint him so they can lick the life out of him.