The placebo effect is already known to be pretty bizarre, but a new study has ramped up the weirdness factor. Researchers have found that people can be trained to believe in a placebo so much, it still works even when they’re told it isn’t real medicine.

This study, published in The Journal of Pain, was conducted by a team from the University of Colorado Boulder (UCB). In it, a ceramic heating element was applied to the forearms of participants, hot enough to cause pain but not too hot that it burned their skin.

The lead researcher, UCB graduate student Scott Schafer, then applied what the participants thought was an analgesic gel, used to relieve pain before applying the heating element on the skin again. In reality, though, the gel was nothing more than Vaseline with blue food coloring, and Schafer simply turned down the heat when it was applied. Each participant was asked medical questions and given information on the drugs to help the illusion. Regular Vaseline, without blue food colouring, was used as a control.

When Schafer set the heat on ‘medium,’ participants reported less pain when they were given the blue Vaseline as opposed to the regular Vaseline – despite the heat remaining constant. After one session, some were told that it was a placebo, and Schafer found that it no longer worked.

However, for those that went four sessions with the blue Vaseline before being told it was a placebo, it was remarkably still effective. It appears that they associated the blue Vaseline with the reduced pain so much that they trusted its effects over Schafer telling them it wasn't real, having felt the benefits regularly. It suggests people can be trained to believe that a placebo works as well as a drug.

"We're still learning a lot about the critical ingredients of placebo effects,” Tor Wager of UCB, senior author on the study, said in a statement.

“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."

The research could be useful in helping treat drug addiction, such as patients in severe pain who have taken strong – and potentially addictive – painkillers. "If a child has experience with a drug working, you could wean them off the drug, or switch that drug a placebo, and have them continue taking it," said Schafer in the statement.