There may be side effects when changing over from glucose metabolism to fat metabolism. These may include headache, fatigue, dizziness, insomnia, difficulty in exercise tolerance, constipation, and nausea, especially in the first days and weeks after starting a ketogenic diet. Breath may develop a sweet, fruity flavor via production of acetone that is exhaled because of its high volatility.
Most adverse effects of long-term ketosis reported are in children because of its longstanding acceptance as a treatment for pediatric epilepsy. These include compromised bone health, stunted growth, hyperlipidemia, and kidney stones.