A day at the spa should leave you feeling healthy, relaxed, and rejuvenated. But some beauty or (so-called) wellness treatments probably aren't worth your money—and some could even be harmful.

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A day at the spa should leave you feeling healthy, relaxed, and rejuvenated. But some beauty or (so-called) wellness treatments probably aren't worth your money—and some could even be harmful. Medi-spas made headlines in October for an extreme case after a Nevada woman died in a cryotherapy chamber, but the reality is there are other potential dangers lurking in this largely unregulated industry.

"There are a lot of mistruths and strange beliefs out there about what's good for you when you go to a spa," says Leena Nathan, MD, a physician in the UCLA Health system. "It's important to remember that just because a service is offered, that doesn't mean it's safe or effective." Before you pamper yourself this holiday season, read up on these treatments you may be better off without.

Skip it: Cryotherapy

The act of exposing parts of the body to freezing temperatures can actually be safe and beneficial, Dr. Nathan says; it's used to remove warts and destroy cancer cells, and some athletes even say it improves muscle recovery.

But recently, whole-body cryotherapy chambers have been touted as a beauty and anti-aging treatment that can supposedly reduce cellulite, even skin tone, and treat skin conditions like psoriasis and acne. For these claims, there's no real evidence, Dr. Nathan says.

On top of that, these machines usually aren't regulated or operated by medical professionals. "Going into a chamber is not recommended from a medical perspective," Dr. Nathan says. "As a physician, I wouldn't be comfortable telling a patient that this is safe."

In Las Vegas, a spa employee was killed after she entered a cryotherapy chamber by herself, after hours; it was later determined that she died from lack of oxygen. Nevada has since issued guidelines about how and by whom cryotherapy should be used, but other states have fewer—if any—regulations.

Proceed with caution: Laser hair removal

As the popularity of laser hair removal has grown, so have the number of treatments gone terribly wrong. When used improperly, the powerful light pulses used to destroy hair follicles can also cause serious skin burns, scarring, and pain.

"Laser hair therapy is safe in the right hands," Dr. Nathan says, "when it's done by a reputable dermatologist, as opposed to a random medi-spa employee." And in fact, 78% of lawsuits filed in 2011 for injury from laser surgeries (including hair removal) were directed at practitioners other than doctors—up from just 36% in 2008, according to a UCLA study published in JAMA Dermatology in 2013. The study also found that 80% of lawsuits filed between 2008 and 2012 were for treatments that took place outside of a traditional physician's office.

Every state regulates laser use differently, with some requiring doctors to be on-site and others with no rules at all. To be safe, Dr. Nathan says, only consider laser therapy performed by or under the direct supervision of a medical professional.

Skip it: Colon cleanses

Think twice about visiting spas that offer colon cleansing—also known as colonic irrigation or colonic hydrotherapy—and definitely stay away from this procedure, Dr. Nathan says. According to a 2011 Georgetown University review of previous studies, the act of flushing water and chemicals through the colon and rectum doesn't have any proven health benefits. And worse: in some people it can cause serious, even life-threatening side effects, from vomiting and cramping to kidney failure.

"The colon really works well all by itself," Dr. Nathan says. "And by pushing water all the way through, you're eliminating a lot of the good flora that your digestive tract needs."

Proceed with caution: Body wraps

Being wrapped up in seaweed, plastic, or another constricting material may not sound too appealing, but many spas tout this practice as a way to purify skin, open pores, and speed weight loss.

When done in a reputable spa, these treatments are generally safe, Dr. Nathan says. But it's possible that wraps that are too tight or left on for too long can cause dehydration and overheating. "There is the potential to lose electrolytes and fluid, so you want to make sure you feel safe the whole time," she adds. Check with the spa staff that you won't be left alone while you're wrapped, and that you can stop the treatment at any time.

If you're looking to shed fat, a body wrap may not be your best investment anyway. Doctors say that any weight lost as a result of a wrap is pure water weight—a temporary change, at best.

Research before you relax

No matter what therapy you're considering, the most important thing you can do is to check out a spa before booking. Even something as simple as a manicure or waxing can have health consequences if a technician isn't well-trained or a facility follows unsafe or unsanitary practices.

"The rules vary by state and by procedure, but spas should display certificates showing that they're qualified to perform whatever it is they're offering," Dr. Nathan says. And if that includes medical procedures like laser therapy, a doctor should be overseeing treatments, if not performing them him or herself.

You can also read online reviews, get recommendations from your own doctor, and ask questions about the staff's education and experience. The extra legwork may raise a red flag—or, at the very least, will reassure you that everything is as it should be. And if that helps you enjoy your spa day without worry, that alone can be worth the effort.