Placebo Effect

Mind over matter.

The placebo effect is a process in which someone thinks that something is healing them, even though it does not have such an effect when the patient is not told what it does, and the belief that they are being healed causes their physical condition to improve (usually just a reduction of the severity of symptoms). This has been used by doctors when a patient is suffering from an easily treatable illness, and is even used to the degree of hosting fake surgeries with no ill effect. The most common use is in drug trials, in which a control group is given a placebo, to compare the effects in case the drug actually is only effective due to the placebo effect, or is even worse. Although not fully understood, certain effects (such as those of a placebo said to contain caffeine) are known to be caused by certain chemicals in the brain (dopamine released in the thalamus for a placebo said to contain caffeine).

The opposite is the nocebo effect, in which someone believes something is hurting them or making them sick, when it's really not. This could even kill them, as Your Mind Makes It Real.

See Magic Feather in terms of the plot. A more realistic version of Clap Your Hands If You Believe.

Film

 * In Euro Trip, two of the characters order brownies at an Amsterdam bakery run by a Dreadlock Rasta. They immediately become stoned beyond belief...until the Dreadlock Rasta calmly informs them that they're not hash brownies.
 * The protagonist of Matchstick Men, who has severe OCD, is given a packet of pills by his psychiatrist that apparently heals him. Around the same time, he starts bonding with his long-lost daughter. He eventually learns that the pills are just soy menopause supplements, and that bonding with his daughter has given his life meaning and helped him overcome his neurosis.
 * The Birdcage: Armand's "Pirin tablets" -- he seems to think they're some kind of powerful anti-anxiety medication; they are in fact Aspirin with two letters scraped off. They seem to do the job, though.
 * In Space Jam, the Tune Squad has completely given up hope of winning their basketball game against the Monstars until Bugs Bunny gives them 'Michael's Secret Stuff', which buffs them up and gives them the confidence to get back in the game. Of course, the real secret is that it is just water.
 * Subverted in The Exorcist. In one scene, a priest douses the demon-posessed protagonist with tap water and claims that it's holy water, but the protagonist screams in pain anyway. Later, though, it's implied that the demon only pretended to be fooled so that the priests would think that the "possession" was purely psychological and wouldn't try to exorcise it. When the priests douse the protagonist with real holy water in the climactic exorcism scene, the screams are real and the water leaves visible burns.

Literature

 * In IT, it is revealed that Eddie Kaspbrak's asthma is psychosomatic, and his medication is a placebo.
 * Inverted by Captain Underpants, who thinks his powers are neutralized if he's sprayed with spray starch.

Live Action TV

 * The Red Green Show had an episode where the lodge members were part of a test-market for an energy bar, making them very active and becoming addicted. When the test batch ran out, they reverted back to normal, only for Harold to reveal all they got was a basic granola bar to gauge product interest, due to the real stuff being too dangerous. Red, taking this as proof of the strength of his mind, salvages one of the test sample bars and tells his wife at home to wait up that night.
 * In an episode of Frasier, Niles eats a normal brownie thinking it's a pot brownie, with the reverse situation for Martin. Hilarity Ensues.
 * One time on MASH they run out of morphine so they give the patients sugar pills telling them it's a strong painkiller. It works.
 * On the B-plot of that episode, they're experiencing a heat wave that has everyone miserable. They give Klinger some of the sugar pills claiming they're some sort of new drug that will allow him to feel cooler. He spends the rest of the episode in his regular uniform while everyone else is wearing undershirts.
 * The Suite Life On Deck: Bailey uses a placebo to raise London's intelligence. Subverted in that after realizing that it's a placebo, London returns to normal. Then she takes another placebo.
 * On The Big Bang Theory, Raj is unable to talk to women unless he's drunk. In "The Terminator Decoupling," the guys are on a train when they discover that Summer Glau is in the same car, and they all try to hit on her. Raj drinks copious amounts of beer before going over to talk with her, and she actually seems to like him. Then Howard walks over and informs him that it was non-alcoholic beer. He clams up and walks away without another word.
 * Referred to when the Myth Busters tested seasickness cures. To be certain that Adam and Grant weren't subconsciously skewing the test results, Jamie gave each of them an "over-the-counter medicine" that was actually a vitamin pill. (Grant fell for the placebo, but Adam got sick just as fast as in the other tests.)
 * In The Next Mutation, the Turtles have a recurring team of foes that once used a magic formula to enhance their abilities. The effect ended when the turtles pointed out the 'magic' was just a placebo.

Western Animation

 * Dogbert has a placebo that works even when the user knows it's a placebo.
 * The Simpsons: The Crazy Cat Lady once regained her sanity thanks to a medication but lost it when she learned it was a placebo.

Real Life

 * Chiropractic adjustments are controversial, owing to the fact that there's little scientific proof that the person doing an adjustment (technically a Doctor of Chiropractic, since they aren't actually medical doctors) is doing anything other than basic decompression of the spine. But because these 'doctors' are very good at convincing people they'll feel better, they do tend to feel better.
 * Acupuncture also has very little proof that it is effective (yes, they are able to do studies with needles that look like they are inserted, but actually are not), but people really believe that it helps them.
 * Therapeutic Touch therapy and it's "Eastern" equivalent, reiki, worked through this effect as well - it's so easily debunked that a 9-year old girl was able to do it, as seen here.
 * Homeopathy may fool many people due to the placebo effect, despite the fact that it would have to utterly violate some of the most basic laws of physics and chemistry in order to work.
 * Pretty much all pseudoscientific "treatments" depend on this. They are mostly (not entirely, but mostly) directed toward the relief of (chronic) pain, which is one of the most incredibly subjective things on the planet to attempt to measure.
 * Guess why the pharmaceutical industry spends more money on advertising than research.