Slendertone is an electrical muscle stimulation (EMS) device; in this case, a belt to be worn around the abdominals that sends mild electrical current to muscles, which mimics the contractions of muscles when you’re exercising. So even if you wear the belt at your desk at work or in the car or running errands or whatever you’re doing in your daily life, the Slendertone is helping to keep you fit.
There’s evidence that Slendertone may help promote muscle tone in already-fit folks—research, studies, and even the US Food and Drug Administration (FDA) says that. But for weight loss, does Slendertone work? Let’s explore.
Slendertone has been in business since 1965 and is headquartered in Hoboken, New Jersey. They have a Better Business Bureau profile opened in 2006, with an A-minus rating (no reviews, one customer complaint they didn’t respond to). Customer contact is (800) 551-2443. 
The Slendertone belt runs about $100 (there are several models) on Amazon, and is covered by a 2-year warranty for defective parts (a few 1-star reviewers complained about the product stopping after a few months because the contacts rusted—the manufacturer promptly responded with the note about the replacement warranty). 
Slendertone.com says that during everyday activities like “carrying shopping bags or keeping your back straight while sitting at your desk, the brain sends signals to the muscles via the nerves, making them contract and relax” and that its toning belt greatly enhances that process with its “clinically-proven EMS technology to emit signals directly from the belt to your abdominal muscles, causing them to contract, work and relax in a similar way.” 
And when you’re actually doing a good workout with a Slendertone on, the claim is it’s among the best, if not the best, way to tone all your abdominal muscles at the same time “including the deep transversus abdominis, which is one of the most effective at flattening the stomach and is extremely tricky to tone with conventional exercise and abs workouts.” Slendertone claims the fierce core you’ll obtain means the most toned and tight abdominals. Plus, Slendertone claims, you will “run further, jump higher, sit straighter, stand taller and feel amazing.” 
Wow. That’s a lot of claim. Is it true?
Slendertone says there’s “no question” the belt will “increase the strength, firmness and tone of people’s abdominal muscles,” based on the statement of an Exercise and Sports Science Ph.D at the University of Wisconsin after a study at the university in 2002. Speaking of that study, let’s talk evidence, facts and science. back to menu ↑
The Science (or Lack Thereof) Behind Slendertone
Right off the bat I wondered how this study— paid for by the manufacturer of Slendertone, performed for the Journal of Sports Science and Medicine by the University of Wisconsin and then reported on by the U.S. National Institutes of Health—could be unbiased. It appears that since the FDA and others were not convinced by the company’s own in-house study, and the company wanted its product to be scientifically evaluated, it was agreed this independent study would be conducted and the results would be whatever they were. And it turns out, the results were a huge win for Slendertone. 
But the study did question,
Why did this study have such positive results when studies using other commercially available (EMS) products found little improvement in many of the same parameters?
The answer probably is related to the strength of the electrically-induced contractions attained by subjects in the current study; in order to improve the strength of a muscle, whether through resistance training or (EMS), the muscle must be overloaded above a critical threshold. 
Slendertone sends a much stronger electrical current than most devices, apparently.
Ultimately, the study definitively states the use of the Slendertone belt
…significantly increased abdominal strength and endurance, decreased waist girth, and improved self-perceived abdominal firmness and tone. The results probably can be attributed to the strength of the electrically induced muscle contractions made possible by the quality of the electrodes utilized in the belt system, as well as the stimulator itself…
and added future studies may be warranted to see if this can help people with lower back pain who can’t do traditional ab exercises. 
So there you have it. Muscle tone is improved and Slendertone may help people like lower back pain sufferers who cannot do ab workouts. But there’s no evidence you lose weight. Per the study:
Even though the subjects felt that their abdomens were stronger and firmer, they didn’t perceive themselves to look any different because they did not lose any subcutaneous fat. 
Right. It turns out while an inch or two may have been lost from the waist due to tightening, the study found “no significant changes in body weight or Body Mass Index (BMI).” 
Okay, so who cares? Stronger, healthier abs is a good thing, right?back to menu ↑
Word on the Street About Slendertone
Remember the first paragraph? This product works best for already-fit people? That’s pretty much the case as evidenced by the 100-plus Amazon reviewers who, for the most part, found the Slendertone effective just as long as one doesn’t already have too much around the middle. 
One anonymous “Amazon Customer” (2017, 5-star), said Slendertone
Literally changed my life. … Haven’t had to do a single crunch or ab exercise. I started out with a flabby, squishy stomach and I currently have a six pack. I use it every day, for two 40 minute sessions, on the highest setting, 99. I’m 5’7 and currently weigh 136lbs. I started in January at 150lbs. So that’s one thing. You need to not be too overweight starting out. … Just had to leave a review and praise this amazing product. I don’t usually don’t do this but just wanted to let everyone know that it’s worth every penny and better than a gym membership.” 
Is this Slendertone reviewer’s experience typical?
“TiffBurn” (2016, 5-star) writes,
“Let’s be clear, this isn’t a weight loss device. If you use it your muscles will get more tone. I don’t think fat can be toned. That being said, I personally think that it helps you build a strong core which helps a lot with doing cardio, which in turn helps you see results. I’m at the max intensity doing the max workout (level 6) and I do sessions 5 times a week. They’ve actually gotten harder to tolerate as time goes on but I find doing something while having the belt on (for me it’s cleaning) helps distract you. 
And just one more for good measure because there is a theme here.
In her Slendertone Belt review, “Celeste Hall” (2015, 5-star), said
I’m about ten pounds overweight, so pretty close to my goal, but my abs are always the hardest part for me to tighten up. My stomach is the first place to gain weight and the last place I loose it.
… I think the belt works best when it can engage close to the muscle, without fat interference. That may be why some people are not seeing the results they expect. I’m looking forward to seeing how it works once I’ve dropped a few more pounds and the belt can properly engage those central abs. 
Her review was deemed helpful by more than a dozen people.
The newest of the Slendertone belts, Abs7, is just as well reviewed with 3.8 stars out of 5 over 69 reviews.
“Ana V.” (2017, 5-star) says,
Awesome! This thing really works. I was so so so skeptical to buy, reviews were mostly positive but so many can be fake…. I still decided to give it a try and return it if it was a waste of money. I have been using it on and off for 3 weeks and I swear to God my tummy feels more sucked in and my abs muscle seem stronger. Make sure you don’t overuse it though or use two EMS devices at the same time. I did and felt funny for a few days. …
An interesting (though not common, apparently) side effect came up on this model that I hadn’t seen before but certainly bears sharing. From “me” (2016, 1-star):
A nasty and unexpected side effect. I am not prone to vertigo or nausea, and have never had motion sickness. … However, two or three hours after using this device, I would get vertigo and have balance problems. … They would gradually disappear over the next 8-24 hours. I had to return the device to Amazon. You have to search hard on the net, but I found references to others having this problem, and I found a few scientific papers on using electrical muscle stimulation to HELP vertigo. … The unit appears to be high quality… I struggled with the rating and eventually decided on one star because this company does not have sufficient help in their manual, quick start pamphlet, or website on medical side effects. … back to menu ↑
The Bottom Line: Is Slendertone Worth a Try?
Depends. If you are already fairly fit without a lot of abdominal fat, this might be good for you. It does help achieve tighter, stronger abs. But it is not a weight loss product and that’s okay—because it never really claimed to be one.
You don’t have to be ripped, but with too much belly fat Slendertone is simply not as effective. If you can’t feel the EMS sending electrical current to your muscles to provoke contractions though your particular tummy, this is not the product for you. Save your money and work on losing belly fat through diet and exercise and then you’ll be ready for Slendertone.