The ketogenic diet—also known as “keto”—has become the latest big thing in weight-loss plans, touted recently by celebs like Jenna Jameson, Mama June, and Halle Berry. The diet involves cutting way back on carbohydrates, to 50 grams a day or less, to help the body achieve a state of ketosis, in which it has to burn fat (rather than sugar) for energy.

Doctors say that keto can be helpful in treating epilepsy; it’s unclear exactly why, but something about a ketogenic state seems to reduce the frequency of seizures. Animal studies have also suggested that the diet may have anti-aging, anti-inflammatory, and cancer-fighting benefits, as well.

But as a general weight-loss plan, keto is more controversial. Some health experts warn against it entirely, citing unpleasant side effects, health risks, and the diet’s unsustainable nature. Even many keto proponents admit that, if the diet’s not done “the right way,” it can be the opposite of healthy.

Here are a few things you should know about the ketogenic diet before you try it as a way to lose weight. Yes, you might drop pounds, but you should also watch out for the following side effects or complications.

The “keto flu”

“Some people report that when they start ketosis, they just feel sick,” says Kristen Kizer, RD, a nutritionist at Houston Methodist Medical Center. “There can sometimes be vomit, gastrointestinal distress, a lot of fatigue, and lethargy.” This so-called keto flu usually passes after a few days, she adds.

Josh Axe, a doctor of natural medicine and clinical nutritionist, estimates that about 25% of people who try a ketogenic diet experience these symptoms, with fatigue being the most common. “That happens because your body runs out of sugar to burn for energy, and it has to start using fat,” he says. “That transition alone is enough to make your body feel tired for a few days.”

You may be able to minimize the effects of keto flu by drinking plenty of water and getting plenty of sleep. Axe, who sells keto-related supplements on his website, also recommends incorporating natural energy sources to battle fatigue, like matcha green tea, organic coffee, or adaptogenic herbs.

Diarrhea

If you find yourself running to the bathroom more often while on a ketogenic diet, a quick internet search will show you that you’re not alone. (Yes, people are tweeting about keto diarrhea.) This may be due to the gallbladder—the organ that produces bile to help break down fat in the diet—feeling “overwhelmed,” says Axe.

Diarrhea can also be due to a lack of fiber in the diet, says Kizer, which can happen when someone cuts way back on carbs (like whole-grain bread and pasta) and doesn’t supplement with other fiber-rich foods, like vegetables. It can also be caused by an intolerance to dairy or artificial sweeteners—things you might be eating more of since switching to a high-fat, low-carb lifestyle.

Reduced athletic performance

Some athletes swear by the ketogenic diet, not just for weight loss but for improved performance in their sport, as well. But Edward Weiss, PhD, associate professor of nutrition and dietetics at Saint Louis University, doesn’t buy it. “I hear cyclists say all the time that they’re faster and better now that they’re on keto, and my first question is, ‘Well, how much weight did you lose?’” he says.

In a recent study in the Journal of Sports Medicine and Physical Fitness, Weiss and his colleagues found that participants performed worse on high-intensity cycling and running tasks after four days on a ketogenic diet, compared to those who’d spent four days on a high-carb diet. Weiss says that the body is in a more acidic state when it’s in ketosis, which may limit its ability to perform at peak levels.

“Just losing a few pounds is enough to give you a huge advantage on the bike, but I’m very concerned that people are attributing the benefits of weight loss to something specific in the ketogenic diet,” Weiss continues. “In reality, the benefits of weight loss could be at least partially canceled out by reductions in performance.”

Ketoacidosis

If you have type 1 or type 2 diabetes, you shouldn’t follow the keto diet unless you have your doctor’s permission and close supervision, says Kizer. “Ketosis can actually be helpful for people who have hyperglycemia issues, but you have to be very mindful of your blood sugar and check your glucose levels several times a day,” she says.

That’s because, for people with diabetes, ketosis can trigger a dangerous condition called ketoacidosis. This occurs when the body stores up too many ketones—acids produced as a byproduct of burning fat—and the blood becomes too acidic, which can damage the liver, kidneys, and brain. Left untreated, it can be fatal.

Ketoacidosis has also been reported in people without diabetes who were following low-carb diets, although this complication is quite rare. Symptoms of ketoacidosis include a dry mouth, frequent urination, nausea, bad breath, and breathing difficulties; if you experience these while following the keto diet, check in with a doctor right away.

Weight regain

Because the keto diet is so restrictive, health experts say it’s not an appropriate diet to follow long-term. (Even Axe says it’s best done for 30 to 90 days, followed by a more sustainable diet plan.) But the problem with that, says Kizer, is that most people will regain a lot of the weight they lost as soon as they go back on carbs.

“It’s an issue with any fad diet, but it seems to be extra common with ketosis,” says Kizer. “When people tell me they want to try it because their friends lost weight, I always tell them, ‘Just watch, I almost guarantee that they’ll gain it all back.’”

These types of back-and-forth weight fluctuations can contribute to disordered eating, Kizer says, or can worsen an already unhealthy relationship with food. “I think this diet appeals to people who have issues with portion control and with binge eating,” she says. “And in many cases, what they really need is a lifestyle coach or a professional counselor to help them get to the bottom of those issues.”

Less muscle mass, decreased metabolism

Another consequence of keto-related weight changes can be a loss of muscle mass, says Kizer—especially if you’re eating much more fat than protein. “You’ll lose weight, but it might actually be a lot of muscle,” she says, “and because muscle burns more calories than fat, that will affect your metabolism.”

When a person goes off the ketogenic diet and regains much of their original weight, it’s often not in the same proportions, says Kizer: Instead of regaining lean muscle, you’re likely to regain fat. “Now you’re back to your starting weight, but you no longer have the muscle mass to burn the calories that you did before,” she says. “That can have lasting effects on your resting metabolic rate, and on your weight long-term.”

Increased risk of heart disease and diabetes

Axe says that, when done right, the ketogenic diet includes lots of vegetables and lean sources of animal protein. In other words, it’s not an excuse to eat butter and bacon—although some people may try to do just that.

That’s why many health experts are concerned about people on the keto diet, especially those who try it without the guidance of a doctor or nutritionist. Doctors say that high-fat diets like this one may raise cholesterol levels, and some studies suggest that they increase the risk of diabetes. Some have even called it a “cardiologist’s nightmare.”

Just this week, a 25,000-person study presented at the European Society of Cardiology Congress in Munich suggested that people on the lowest-carb diets had the highest risk of dying from cancer, cardiovascular conditions, and all other causes. Another study, published this month in the Lancet, also found that people who followed diets that were low in carbs and high in animal proteins had a higher risk of early death compared to those who consumed carbs in moderation. (The opposite was true, however, for low-carb dieters who opted for plant-based proteins over meat and dairy.)

“Whether you’re in the paleo camp or the keto camp or the vegan camp, everyone agrees that we want to have a nutrient-rich diet,” Axe says: “Lots of vegetables, herbs, spices, and plant-based sources of fat and protein, too.”

“If you’re not doing that, you’re promoting disease in the body—it’s that simple,” Axe says. (And yes, that’s true even if you still lose weight in the beginning.) “If you’re just going to eat butter and bacon,” he adds, “I’d rather you not do keto at all.”