Hypnosis has weirdly positive results in weight loss studies. That doesn't mean it works.

Dear Julia: Does hypnosis for weight loss really work? It seems hard to believe.

The idea behind hypnotherapy is fairly simple: A trained hypnotist puts a patient in a trancelike state that makes her ultra-responsive to suggestion. Proponents of this mysterious treatment claim that it can work wonders, helping people permanently change their behavior to lose weight or quit smoking.

But you're right to be skeptical. The actual evidence on hypnotherapy suggests it has a very modest impact, at best, in helping people shed pounds. One comprehensive review in the International Journal of Obesity examined the evidence on hypnotherapy programs designed to help people lose weight. That review found that the participants typically only lost a couple of pounds, which is a very small effect size considering the time and money invested.

There's also an odd twist here: It's not even clear that the hypnotherapy itself was the reason for the weight loss in these studies. It might just be an artifact of how the studies are designed.

To see why, we have to walk through the recent history of hypnotherapy studies.

Our story starts with a 1995 meta-analysis, in which researchers crunched the data from 18 different studies (meta-analyses are usually considered more reliable than individual studies). These studies all looked at the effects of cognitive behavioral therapy as a weight loss treatment — and then compared it with the effects of CBT plus hypnosis. The idea was that hypnosis might make CBT more effective.

The initial results seemed promising. After about six months, according to the meta-analysis, the hypnosis group lost a little more than 20 pounds, while the CBT-only group lost about 10 — and amazingly, both groups kept off most of the weight even by the 24-month mark:

The Journal of Consulting and Clinical Psychology

The only catch was that these results seemed way too good to be true. After all, it's very rare that people undergoing any sort of weight loss treatment manage to keep the weight off for an extended time period.

So a different group of scientists began reanalyzing the data in this study — a beautiful example of how science should work. In their 1996 reanalysis, these researchers found that the addition of hypnotherapy only led to a small additional reduction in weight (around one pound), and that the number shrank even further when a study with problematic methods was removed from the sample. This new finding — that hypnotherapy only had modest power — was also more consistent with the other available studies on the question. And unlike the original 1995 study, the reanalysis fit better with the large body of existing obesity literature showing that sustained weight loss is extremely difficult.

Follow-up studies on hypnotherapy also found modest effects. One 1998 randomized trial looked at the effects of a diet advice program with and without two different forms of hypnotherapy in 60 obese patients. After three months, all groups had lost around 2 to 3 percent of their bodyweight. After 18 months, only one of the two hypnotherapy groups (in which the therapy focused on stress reduction) was still keeping the weight off. Even here, the weight loss was only about 8 pounds, which the researchers deemed "clinically insignificant" given how much weight the patients needed to lose to treat their sleep apnea.

So that's where we are today: Studies on hypnotherapy consistently find that people lose very modest amounts of weight. Is this proof that hypnotherapy works? Not necessarily.

For one, the results might be a reflection of the fact that it's difficult to study the treatment in a scientifically optimal way. None of these studies were blinded, for example. That's the gold standard for research — when participants don't know what sort of treatment they're actually receiving. ("Double blinding" means the doctors don't know either.) As you can imagine, blinding participants would be pretty difficult for a trial on hypnosis. But this means that the hypnotized in any study may be vulnerable to a placebo effect. (And research has show that more dramatic interventions — say, needles as opposed to pills — have more dramatic placebo effects.)

Second, consider that the impact of hypnosis seems to be tiny. If it worked, shouldn't it have more impressive results? Hypnotherapy also hasn't been shown to be effective in helping to treat other problems, like fibromyalgia pain or smoking. So while we can't say for sure, we should at least be skeptical.

Should you try hypnosis? Well, there's no harm to it. But considering hypnotherapy sessions can run at least $100, and research has found insignificant results, you might want to save your money for something else.

Welcome to Dear Julia, a weekly column where readers can submit everyday health questions. Which over-the-counter painkillers work best? Is it better to run or walk for exercise? How much harm does frequent flying do to your body? Julia Belluz will sift through the research and consult with experts in the field to figure out how science can help us live happier and healthier lives.

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