While a ketogenic diet is considered to be safe for most people, it does have some potential drawbacks.
Some people experience mild adverse effects when starting a ketogenic diet, and this is sometimes referred to as “keto flu”.[1] The symptoms include nausea, abdominal discomfort, vomiting, fatigue, and headache. These symptoms are likely to resolve within a few weeks.[2] Due to a very low carbohydrate intake leading to a low fiber intake, constipation is also a possibility.[3]
Another potential drawback is the effect that a diet high in fat can have on lipid profiles. This effect varies between individuals, but in some people the ketogenic diet can cause a significant increase in total cholesterol and LDL cholesterol. This could have a serious effect on the risk of cardiovascular disease, and care should be taken in recommending this diet to people with high cholesterol and other cardiovascular risk factors.[4][5][6][7][8][9][3]
The high fat and low fiber intake may increase the risk of developing kidney stones, which are extremely painful and can have serious consequences.[1] The restrictive nature of the ketogenic diet could lead to vitamin and mineral deficiencies in some people.[1]
Another significant drawback to the ketogenic diet is that it can be very difficult to adhere to such strict dietary limitations. A decrease in adherence over time may contribute to the decrease in effectiveness seen in the long term after initial improvements in the short term.[3]
In people who are prone to hypoglycemia (low blood sugar) and people who use insulin, the ketogenic diet should be started with caution. Close monitoring of glucose levels is important, because a sudden drop in blood glucose can be dangerous. Furthermore, the ketogenic diet is not safe for people with certain medical conditions, including porphyria, pancreatitis, liver failure, and disorders of fat metabolism.[1][10]