Keto flu, keto breath, keto crotch… What’s next? Unfortunately, new side effects of the keto diet continue to emerge.

People are now breaking out in full-body, painful rashes after going on the extremely low-carb diet. And perhaps the worst part is that this “keto rash” can leave brown marks on the surface of the skin that sticks around for life.

One of the most popular diet trends of this decade, the idea behind the keto diet is that restricting carb intake to an extremely low number while eating large amounts of fats will trick the body into entering ketosis, a state in which the body burns fats for fuel. Ketosis is difficult to maintain, but once it is achieved, the body can react in strange (and in this case painful) ways.

If you search “keto rash” on Reddit, you’ll find dozens of dieters begging for help with the condition, sharing anecdotes of excruciating itching and discomfort. The rash tends to start near the chest and spread, reaching every part of the body, including the scalp.

“I noticed a rash across my left boob within 3 days,” one Reddit user recalled. “By day 14 (Saturday), my rash was up and down both of my boobs, in my cleavage, along my bra line in the back and all over my neck, into my hairline. It was PAINFUL, burning, itching. I was insanely irritable. It looked horrific.”

One Reddit user, ketoko, said they had been wearing gloves to bed to prevent from scratching throughout the night. “I’m starting to develop scabs and blisters and it just hurts really bad,” ketoko said in a post.

Even once the rash subsided, some found that the marks stuck around for good.

“I spent years trying to overcome insecurities. I decided to try keto to finally change for the better,” user donut-peach said, “but now I have a totally different problem that prevents me from showing my skin … it was spreading too rapidly and left behind discolouration. I now am off keto and have hideous marks all over my chest, shoulders, neck, and back.”

The condition is called prurigo pigmentosa. Existing scientific literature shows a link between ketosis and the rash, though studies show other intervening factors (such as sunlight, extreme heat, allergens and sweating) may worsen the condition. Going on the keto diet is not the only way humans can enter ketosis. Conditions such as diabetes mellitus and anorexia nervosa can also lead to this state, as well as fasting and dieting.

A 2018 case study showed that prurigo pigmentosa can only really be treated by eating a lot more carbs — i.e., quitting keto. Other options, such as topical creams and anti-inflammatory drugs, may temporarily relieve symptoms — but if you don’t quit the diet, the rash is likely to return in full force once you stop using them.

“There is no dietary treatment option to successfully treat the rash without sacrificing ketosis,” the study reads. And once the rash disappears, there is no known method of getting rid of the dark brown spots prurigo pigmentosa leaves behind.

Not everyone on the keto diet experiences this symptom. But you should know if there are serious risks to a diet before you try it. Keto isn’t the only diet with drawbacks — these other popular diets have hidden side effects, as well.