The most common side effects of a keto diet are the “keto flu” in the first week or two (headache, tiredness, irritability, dizziness and brain fog), constipation, bad breath, leg cramps, trouble sleeping and a drop in exercise performance. Most of these ease within a few weeks and are helped by drinking enough water, getting enough salt, potassium and magnesium, and eating plenty of low-carb vegetables. Longer-term risks include higher LDL cholesterol in some people, kidney stones, nutrient shortfalls and, rarely, a skin rash. A few side effects are serious, such as low blood sugar or ketoacidosis in people taking certain diabetes medicines, so talk to your doctor before starting keto, especially if you take regular medication.
Side effects at a glance
| Side effect | When it happens | What helps |
|---|---|---|
| Keto flu | First 2 to 14 days | Water, salt, electrolytes, easing in |
| Constipation | First weeks, sometimes ongoing | Fiber from vegetables, seeds, water |
| Bad breath | Early weeks | Water, oral hygiene, usually fades |
| Leg cramps | Early weeks | Electrolytes, especially magnesium and potassium foods |
| Poor sleep | First weeks | Avoid late caffeine, eat enough |
| Lower exercise performance | First weeks | Ease off intense training while adapting |
| Heart palpitations | Early weeks | Fluids and electrolytes; see a doctor if persistent |
| Higher LDL cholesterol | Weeks to months | Choose unsaturated fats, get tested |
| Kidney stones | Months to years | Fluids, medical advice if at risk |
| Hair thinning | 2 to 4 months after rapid weight loss | Enough protein and calories, usually temporary |
| Keto rash | Weeks | See a doctor or dermatologist |
| Low blood sugar or ketoacidosis | Any time, with some medicines | Medical supervision |
Short-term side effects
Keto flu
As your body switches from using carbohydrate to fat and ketones for fuel, you lose water and salt quickly, which can cause headaches, tiredness, dizziness, nausea, irritability and difficulty concentrating. It usually lasts a few days to two weeks. Drinking more water, adding a little salt to food or having broth, and eating potassium-rich foods such as avocado, spinach and mushrooms usually helps. Our guide to keto flu covers it in detail, and our guide to keto electrolytes explains how much salt, potassium and magnesium you need.
Constipation
Cutting out grains, beans and most fruit lowers fiber intake, and less water in the diet makes it worse. Adults are advised to eat 30 g of fiber a day, according to the NHS, which is hard on strict keto. Non-starchy vegetables, chia and flaxseed, avocado, nuts and plenty of fluids help. Our guide to fiber on keto lists the best sources.
Bad breath
Some people notice a fruity or nail-polish smell on their breath. It comes from acetone, one of the ketones, which is breathed out. It often fades after a few weeks. Drinking water, brushing your tongue and chewing sugar-free gum help.
Leg cramps
Night-time cramps are common in the early weeks, usually linked to fluid and electrolyte losses. Eat magnesium- and potassium-rich foods such as leafy greens, nuts, seeds and avocado. Talk to your doctor before taking supplements, especially if you have kidney problems or take medicines.
Trouble sleeping
Some people sleep poorly for the first couple of weeks. Avoid caffeine after midday, don’t cut calories too hard, and make sure you’re eating enough. It usually settles. Our guide to foods for sleep has general tips.
Reduced exercise performance
High-intensity exercise relies on carbohydrate, so sprints, heavy lifting and fast running may feel harder for several weeks. Endurance at a steady pace often recovers as you adapt. Our guide to keto and exercise explains how to adjust.
Heart palpitations and dizziness
Fluid and salt losses can lower blood pressure and cause a faster heartbeat or lightheadedness, especially when standing up. More fluids and salt usually help. If palpitations persist, or you have chest pain or fainting, seek medical help.
Longer-term side effects
Higher LDL cholesterol
LDL cholesterol rises in some people on keto, sometimes a lot, especially with plenty of butter, cream, cheese and processed meat. In a randomized trial comparing keto with a Mediterranean diet, LDL rose by about 10% on keto and fell by about 5% on the Mediterranean diet, and keto diets were lower in fiber and several nutrients. Ask your doctor for a cholesterol test before and a few months after starting. Our guide to keto and cholesterol explains what to do.
Kidney stones
Long-term ketogenic diets are linked with a higher risk of kidney stones, particularly in children treated for epilepsy. Drinking plenty of fluids helps. People with a history of stones or kidney disease should talk to their doctor first. Our guide to keto and kidney stones has the details.
Nutrient shortfalls
Strict keto can be low in fiber, vitamin C, folate, magnesium, potassium and calcium. Planning meals around vegetables, nuts, seeds, fish and dairy helps. Our guide to nutrient deficiencies on keto explains which to watch.
Hair thinning
Rapid weight loss on any diet can trigger temporary hair shedding a few months later, a condition called telogen effluvium. DermNet explains that it usually recovers on its own within months. Eating enough protein and not cutting calories too far helps reduce the risk.
Keto rash
A rare itchy rash called prurigo pigmentosa can appear on the chest, back and neck when people start keto or fast. It can leave darker patches as it heals. See a doctor if you develop an unexplained rash. Our guide to keto rash covers it.
Gout flares
Uric acid levels can rise in the first weeks of keto, as ketones compete with uric acid for removal by the kidneys. People with gout may have a flare. If you have gout, ask your doctor before starting; the US National Institute of Arthritis and Musculoskeletal and Skin Diseases explains how gout is managed.
Menstrual changes
Some women notice changes to their cycle, often linked to rapid weight loss or eating too little. If periods stop or change a lot, see your doctor.
Serious risks that need medical advice
- Low blood sugar: cutting carbs while taking insulin or sulfonylureas can cause hypoglycemia. The NHS describes the symptoms, which include sweating, shaking, hunger and confusion. Doses usually need adjusting before you start.
- Ketoacidosis: people with type 1 diabetes and those taking SGLT2 inhibitors for type 2 diabetes can develop ketoacidosis, a medical emergency. Signs include feeling or being sick, stomach pain, fast breathing and confusion. The NHS explains when to get urgent help. Our article on keto and type 2 diabetes covers the risks.
- Low blood pressure: blood pressure medicines may need reducing as you lose weight and fluid.
- Pregnancy and breastfeeding: keto isn’t recommended without specialist advice. Our guide to keto during pregnancy explains why.
How to reduce side effects
- Ease in by cutting carbs over a week or two rather than overnight.
- Drink plenty of fluids and add a little salt to meals, unless you’ve been told to limit salt.
- Eat lots of non-starchy vegetables for fiber, potassium and magnesium.
- Choose mostly unsaturated fats from olive oil, nuts, seeds, avocado and oily fish.
- Eat enough. Very low calorie intake makes tiredness, hair loss and cramps more likely.
- Get check-ups, including blood tests. Our guide to keto blood tests lists them.
Common questions
How long do keto side effects last?
Most early side effects settle within 2 to 4 weeks. Constipation and cholesterol changes can persist and need ongoing attention.
Is it normal to feel tired on keto?
In the first one to two weeks, yes. If tiredness lasts longer, check that you’re eating enough and getting enough salt and fluids, and see your doctor if it continues.
Are keto side effects dangerous?
Most are uncomfortable rather than dangerous. Low blood sugar and ketoacidosis in people on certain medicines are serious, which is why medical advice matters before starting. Our guide to whether keto is safe long term covers the bigger picture.
Safety note: Keto isn’t suitable for everyone. If you have diabetes, kidney or liver disease, take regular medication, or are pregnant or breastfeeding, talk to your doctor before cutting carbs this far (why it matters).
This article is general information, not medical advice. Talk to your doctor or a registered dietitian before making big changes to how you eat, especially if you have a health condition, take regular medication, or are pregnant or breastfeeding.
