The ketogenic diet, which is heavy in fat and skimpy on carbs, has become a popular eating method geared toward weight loss; plans typically recommend getting 70%–80% of calories from fat. But it can also leave people feeling like they have been hit by a truck.

When a study in Frontiers in Nutrition evaluated comments made in 43 online forums dedicated to keto eating plans, it identified moderate-to-severe fatigue, headache, dizziness, constipation and mental fog as typical short-term effects. This cluster of symptoms is otherwise known as the “keto flu.”

When your body is required to shift into a completely different metabolic state than it’s used to, it can respond by making you feel miserable. That said, people’s symptoms generally peaked in the first week of the diet, then tapered off over the next few weeks. So, if you have clients eager to give the keto approach a go, be sure they’re prepared for a hard landing. There is also a concern that keto followers may fall short on various vitamins, minerals and fiber, plus the diet’s long-term sustainability remains in question.

See also: What’s the Word on Keto for Weight Loss?