You have no doubt heard of the placebo effect. It's what happens when you take a medication or a treatment that you believe will help you, when in reality it has no proven therapeutic effect. But it still makes you feel better. It's a scarcely understood part of medicine that's basically all about getting real results with fake input.

I'm Michael Aranda filling in for Hank Green and I promise this won't hurt a bit.

Scientists have known since the 1700s that placebos can sometimes provide real effective treatment, but exactly how it works is not well understood.

What we do know is that placebo can be such an effective medical head-fake that in the U.S. federal law requires that double-blind placebo tests be used in the trials of any new drugs. This means that a new drug must beat its corresponding placebo in at least two clinical trials to become F.D.A. approved. Which as it turns out is harder than it sounds.

Placebos have been proven to alleviate depression, anxiety, pain, and insomnia; they elevate mood, and sharpen thinking. They work not just from the belief that the drug you're using is real, but also from a subconscious trust in doctors.

There's some classic Pavlovian conditioning at work here. You remember Pavlov; the guy that rang a bell whenever he fed his dog, and then eventually just the sound of the bell would cause the dog to drool. It's kind of like how kids are obsessed with band-aids and automatically feel better when you put one on their 'owies'. We expect to feel better after being treated and that's actually where the name comes from. 'Placebo' is Latin for 'I shall please'. Basically it enshrines the power of 'everything's going to be alright'.

It's not just your mental perception, but also your body's physiological reaction to that anticipation which actually activates immune responses that limits the secretion of damaging stress hormones.

Studies have linked the placebo effect to the brain's natural production of pain relieving endorphins. The expectation of feeling better actually causes the brain's endorphin-receiving opioid receptors to fire up, activating its own pain relief system. These are the receptors that opiates and some other pain-killers activate to relieve pain.

Interestingly, another study found that advanced Alzheimer's patients experienced less pain relief from both analgesics and placebos. The disease had damaged their opioid receptors so they couldn't benefit from the anticipation of treatment. Real or fake.

But the power of thought can work against you too. The placebo effect has an equally powerful dark-side: the nocebo effect.

Nocebos showcase the negative effects of thinking or anticipation. It's what's at work when say, sugar pills, cause real negative side-effects. For example: in one medical trial of a drug for fibromyalgia, a syndrome marked by chronic wide-spread pain, over 10% of volunteers given a placebo dropped out of the study because of the side-effects like nausea and dizziness that they'd been warned about, even though the drug was inert. The mere suggestion that they might feel side-effects actually caused the side-effects.

The nocebo effect can also occur in patients taking real drugs. This means if a doctor lists possible side-effects, a patient is more likely to experience them. During trials for a drug designed to reduce enlarged prostate, half of the patients were told that the drug may cause erectile dysfunction, while the other half were not informed. Only 15% of the uninformed men reported the side-effect, while a whopping 44% of the men who weren't had trouble in the bedroom.

In the end we're still not sure how placebo and nocebo responses work. It's likely the mechanisms vary depending on the circumstances. But Ted Kaptchuk, director of Harvard University's Program in Placebo Studies & the Therapeutic Encounter suspect the bottom line might be pretty simple. Real drugs are important, but compassionate care and the act of trust in your caregiver are really critical. Kaptchuk believes that the placebo effect has more to do with the attention, warmth, eye-contact, and empathy given by a physician, rather than the belief that a pill is real. In that case placebos are all about the ritual of medical care and the trust we put into it.

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