The placebo effect is real - even if you know the treatment you've been given has no medical value, research has concluded.
Scientists found patients given a fake painkiller continued to feel benefits from it after they were told it was not genuine.
But there was a catch. For in order to feel these effects, the subjects had to be conditioned into thinking the treatment was real - and given enough time for this belief to become ingrained.
Those told it was fake after just one session did not continue to experience pain relief, the US study found, but those told after four sessions still felt the benefits.
Senior author Tor Wager, of the University of Colorado Boulder, said: "We're still learning a lot about the critical ingredients of placebo effects. What we think now is that they require both belief in the power of the treatment and experiences that are consistent with those beliefs.
"Those experiences make the brain learn to respond to the treatment as a real event.
"After the learning has occurred, your brain can still respond to the placebo even if you no longer believe in it."
To test their theory, the scientists applied heat to the 54 participants' forearms - enough to make them experience a strong sensation of pain without burning the skin.
Co-author Scott Schafer said some volunteers had to be turned away at this stage as they had higher than usual pain thresholds due to working in restaurants and being used to carrying hot plates of food. After using heat of up to 47.5C on the participants, the scientists applied what the subjects thought was a gel containing a painkiller. In fact, this was merely Vaseline with blue food colouring in an official-looking pharmaceutical container.
Then, unbeknown to the participants, the researchers turned down the heat so it felt like the treatment was working. To help create the illusion that they were receiving a real painkiller, the subjects were asked to read drug forms and indicate whether they had liver problems or were taking other medication.
The scientists found four sessions were enough to train the participants' brains to get relief from the fake drug.
Mr Schafer said: "They believed the treatment was effective in relieving pain. After this process, they had acquired the placebo effect.
"We tested them with and without the treatment on medium intensity. They reported less pain with the placebo."
He said the findings could open doors to new ways to treat drug addiction, or aid in pain management for adults and children who are recovering from surgery and reliant on painkillers.
"If a child has experience with a drug working, you could wean them off the drug, or switch that drug to a placebo and have them continue taking it," Mr Schafer added. He said it is known that placebos can induce the release of pain-relieving substances in the brain, and called for more research.
The study, also co-authored with Luana Colloca of the University of Maryland, Baltimore, is published in The Journal of Pain.
- Daily Mail