It’s important not to confuse ketosis with ketoacidosis. Ketoacidosis is an extremely abnormal form of ketosis and yes it is dangerous.
The difference between the two conditions is a matter of volume and flow rate.
Benign nutritional ketosis is a controlled, insulin-regulated process which results in a mild release of fatty acids and ketone body production in response to either a fast from food or a reduction in carbohydrate intake.
Ketoacidosis is driven by a lack of insulin in the body. Without insulin, blood sugar rises to high levels and stored fat streams from fat cells. This excess amount of fat metabolism results in the production of abnormal quantities of ketones. The combination of high blood sugar and high ketone levels can upset the normal acid/base balance in the blood and become dangerous. In order to reach a state of ketoacidosis, insulin levels must be so low that the regulation of blood sugar and fatty acid flow is impaired.
This is why some medical professionals describe ketones as dangerous especially when associated with Type 1 diabetes. Type 1 diabetics have pancreatic damage which results in a complete lack of insulin production. Without that message from insulin, large quantities of fatty acids flow out of the fat cells and are broken down in the liver into a ketone body called acetoacetic acid which is then converted to two other circulation ketone bodies, beta-hydroxybutyrate and acetone.
This is ketosis, but an unrestrained, abnormally excessive ketosis.
Nutritional ketosis associated with a properly formulated ketogenic diet is not dangerous because it is regulated by insulin within the body.
It’s simply the metabolic process of burning your own body fat for fuel, and unless you are diabetic and lacking insulin, or you are a raging alcoholic, it is perfectly safe.
Essential Extended Readings and Research:
More reading on the difference between Ketosis vs Ketoacidosis and research on Type 1 diabetics.
And more on “Is ketosis dangerous?” by from Dr Peter Attia, a physician with extensive knowledge about ketosis.