A patient's belief that a drug will not work can become a self fulfilling prophecy, according to researchers.
They showed the benefits of painkillers could be boosted or completely wiped out by manipulating expectations.
The study, published in Science Translational Medicine, also identifies the regions of the brain which are affected.
Experts said this could have important consequences for patient care and for testing new drugs.
Heat was applied to the legs of 22 patients, who were asked to report the level of pain on a scale of one to 100. They were also attached to an intravenous drip so drugs could be administered secretly.
The initial average pain rating was 66. Patients were then given a potent painkiller, remifentanil, without their knowledge and the pain score went down to 55.
They were then told they were being given a painkiller and the score went down to 39.
Then, without changing the dose, the patients were then told the painkiller had been withdrawn and to expect pain, and the score went up to 64.
So even though the patients were being given remifentanil, they were reporting the same level of pain as when they were getting no drugs at all. ...
via BBC News - Negative experiences can stop painkillers working.
Some people can have teeth removed with only self hypnosis.
Dentist Bhavin Bhatt pulls Leslie Mason's teeth without aneasthetic as hypnotist John Ridlington offers moral support
Brave Leslie Mason used mind over molar to have his teeth removed without anaesthetic - using only hypnosis as a painkiller.
The father-of-seven needed two rotten teeth and their roots removed in a procedure which normally requires a general anaesthetic in hospital.
But he couldn't afford the £400 cost of private treatment so offered to be a guinea pig for a new experimental type of hypnosis.
Mr Mason remained conscious with his eyes open throughout the two-hour procedure - and felt nothing more than 'a bit of a sting'.
The 54-year-old said: 'It was incredible. There is no worse pain than that inflicted by dentists but I didn't feel any.
'The dentist had to dig away at the rotten roots that were right up into my jaw. There isn't anything I wouldn't have done under hypnosis now. It's incredible.' ...
The operation was performed by Dr Bhavin Bhatt, who runs the Smile and Wellbeing dental practice in Bishop's Stortford, Hertfordshire.
He said: 'The hypnosis was 100 per cent effective. We're now exploring the possibility of offering tooth transplants under hypnosis.'
Studies have shown that using hypnosis instead of anaesthetic can reduce recovery time after surgery.
It also removes the chance of possible side-effects from anaesthesia.
But the technique's long association with stage acts has kept it at the margins of mainstream medicine.
Hypnoanaesthesia, where the patient enters a deep trance state and is told he will feel no pain, has been used to help burn victims manage their agony and ease fears over surgery and childbirth.
The British Society of Medical and Dental Hypnosis said that even heart operations had been carried out under hypnosis.
In April, hypnotherapist Alex Lenkei had an 83-minute operation without anaesthetic on his arm to treat osteoarthritis.
Afterwards, he revealed he had been aware of his surgeon making a four-inch incision into his wrist and chiselling the bone to remove a tendon.
He added: 'I heard everything he was saying to his assistants and anaesthetist but there was no gossip. It was a shame - I was hoping to hear something juicy.'
Read more: dailymail
VANESSA ALLEN - A hypnotherapist had an 83-minute operation on his arm with no anaesthetic. At one point, Alex Lenkei even heard the surgeon say: "Can I have the saw, please?" Because he had put himself into a hypnotic trance, however, he said he felt no pain as the doctors chiselled out a walnut- sized chunk of bone from his wrist.
Mr Lenkei, 61, had the operation to treat his painful osteoarthritis. He said: "The results have been amazing. I feel ace." The hypnotherapist amazed doctors by asking how things were going halfway through the surgery, at Worthing Hospital in West Sussex on Wednesday. He said he could hear his surgeon talking as he made a four-inch-long cut into his right wrist and chiselled the bone to move a tendon.
The married father-of-one, who lives in the town, said: "It took me about 30 seconds to put myself under and I wasn't aware of any part of my body apart from my arm. "I could feel the surgeon pulling and manipulating me - then I heard the cracking of bones.
"I heard him say, 'Can I have the saw please?' and imagined him holding this great big thing in his hand. But fortunately he then said 'I think we'll use the smaller one.'" "He used a hammer and chisel at one stage and I could hear him hammering away at the bone. ...
However, it works only on those susceptible to the technique.
Mr Lenkei has taught students at the Royal College of Nursing how to induce hypnoanaesthesia and said he would consider undergoing further operations without anaesthetic.
Read more: dailymail
Thirty seconds!? It is amazing to me that the doctors would go ahead with the surgery