Keto Side Effects: What Is Normal and What Is Not
Keto flu is only the first week. These are the side effects people ask about after that.
Short answer: Headache, tiredness, irritability, cramps and constipation are the most common, usually grouped as keto flu.
Digestive changes
- Constipation: common when fibre drops with grains and fruit. Add low-carb vegetables, chia or flax, fluids and magnesium. Full plan: keto constipation relief.
- Diarrhoea: often linked to a sudden jump in fat, MCT oil, or sugar alcohols. Increase fat gradually and cut back on "sugar-free" products.
Breath and body odour
Acetone breath is common in the early weeks and tends to fade. Brushing, water and sugar-free gum help.
Muscle cramps
Often an electrolyte issue, especially magnesium and sodium. See keto electrolytes.
Hair shedding
Some people notice shedding two to four months after a rapid change in diet or weight. This is generally temporary. Make sure protein and overall energy intake are adequate.
Heart palpitations
Usually linked to fluid and electrolyte shifts early on. Persistent palpitations, chest pain or fainting need medical assessment.
Cholesterol changes
LDL cholesterol rises noticeably in some people on keto. Get a blood test before and a few months after starting, and discuss results with your doctor.
Kidney stones
Reported in some people on long-term ketogenic diets. Fluids and rotating high-oxalate foods help reduce risk.
Keto rash
A small number of people develop an itchy red rash on the chest, back or neck within a few weeks of starting keto. It can leave net-like brown marks as it fades. Its medical name is prurigo pigmentosa. It is rare, has been linked to ketosis (including from fasting), and its exact cause is not yet understood. Reported cases have often improved when some carbohydrate was added back to the diet, and doctors can prescribe treatment. If you develop an unexplained rash, see a doctor or dermatologist rather than guessing.
For the bigger picture on long-term safety, see is the keto diet safe?
Red flags — stop and seek medical advice
Vomiting, severe abdominal pain, confusion, rapid breathing, fainting, or a ketone reading that keeps rising while you feel unwell.
Frequently asked questions
What are the side effects of keto in the first week?
Headache, tiredness, irritability, cramps and constipation are the most common, usually grouped as keto flu.
What is keto rash?
Keto rash (prurigo pigmentosa) is a rare, itchy red rash, usually on the chest, back or neck, linked to ketosis. It often improves when some carbohydrate is reintroduced, and a doctor can treat it — see a dermatologist if you develop an unexplained rash.
Does keto raise cholesterol?
In some people LDL cholesterol rises significantly. Testing before and after a few months lets you and your doctor see your own response.
This guide is general information for healthy adults, not medical advice. See our medical disclaimer.