Intermittent fasting is generally well tolerated by healthy adults in short-term studies, but it is not safe for everyone. Some people should not fast at all. Others should only fast with guidance from their healthcare provider. And most people who fast will at some point notice a symptom and wonder whether it is normal adjustment or a signal to stop.
This page covers who should not fast, who needs medical supervision, the warning signs that should end a fast immediately, how to tell ordinary adjustment symptoms from danger, and practical ways to lower your risk if you do choose to fast.
Table of Contents
Absolute Contraindications: Do Not Fast
For these groups, intermittent fasting carries clear risk that outweighs any potential benefit. The recommendation is straightforward: do not fast, including short daily windows like 16:8.
- Pregnancy and breastfeeding. Caloric and nutrient demands are elevated. Restricting eating windows reduces total intake for many people, even unintentionally. Risks include impaired fetal growth and reduced milk supply.
- Children and adolescents (under 18). Growth and development require consistent caloric and nutrient intake. There is no demonstrated benefit that outweighs the risk of disrupted growth or disordered eating patterns.
- Active or past eating disorder. Anorexia nervosa, bulimia nervosa, binge eating disorder, and other specified feeding/eating disorders. Fasting structures can reinforce restriction patterns, trigger relapse, or escalate disordered behaviour. This is true even of mild protocols like 16:8.
- Underweight (BMI under 18.5). Caloric deficits in this state can cause hormonal disruption, muscle loss, and impaired immune function.
- Type 1 diabetes (without medical supervision). Risk of severe hypoglycaemia and diabetic ketoacidosis. Fasting with type 1 diabetes should only ever be considered with close supervision from your diabetes care team — never on your own.
- History of severe hypoglycaemia. Including reactive hypoglycaemia or hypoglycaemia unawareness from any cause.
Why “just 16:8” still counts
People sometimes assume short daily fasting windows are exempt from these warnings. They are not. The risk in pregnancy, eating-disorder history, and underweight states is structural — restricting when you eat tends to also reduce how much you eat, and the structure itself can reinforce harmful patterns.
Relative Contraindications: Medical Supervision Required
Fasting may be possible for people in these groups, but only with input from a healthcare provider who knows your medical history — ideally the clinician who manages the relevant condition or prescribes your medication.
Type 2 diabetes on medication
Particularly insulin and sulfonylureas (such as gliclazide, glipizide, glimepiride, and glyburide). These medications can cause hypoglycaemia when food intake drops, and doses often need to be reduced or rescheduled. SGLT2 inhibitors carry a risk of ketoacidosis during fasting. Metformin is generally lower-risk, but any glucose-lowering medication should be discussed before fasting. Never change doses on your own. See fasting and diabetes.
Blood pressure medication
Fasting and the weight loss that may come with it can lower blood pressure, and fluid and salt shifts during longer fasts can add to this. Combined with blood pressure medication (especially diuretics), this can cause dizziness, lightheadedness on standing, or fainting. Doses may need adjustment by your prescriber.
Anticoagulants and other narrow-window medications
Some medicines, including warfarin, levothyroxine, and certain antibiotics, antifungals, and HIV medications, are affected by food or by changes in diet. Discuss timing with your prescriber or pharmacist. See fasting and medications.
Gout
Fasting can temporarily raise uric acid levels and may trigger gout flares in some people, particularly during longer fasts.
Gallstones
Rapid weight loss and long gaps between meals are associated with a higher risk of gallstones and gallbladder symptoms in susceptible people.
Chronic kidney disease, liver disease, advanced cardiovascular disease
Metabolic and electrolyte shifts during fasting can be poorly tolerated. Medical guidance is essential.
Older adults (65+)
Not a contraindication in itself, but the margin for error is smaller. Age-related muscle loss (sarcopenia), taking several medications, and a weaker sense of thirst all make problems more likely. Shorter, gentler schedules and enough protein matter more at this age. See fasting after 50.
Recent surgery or active infection
Healing and immune function depend on getting enough calories and protein. Wait until you have recovered, and check with your healthcare provider after surgery.
Warning Signs That Mean Stop Immediately
The following symptoms during a fast are not adaptation discomfort. They are signals to eat now and, in several cases, seek medical attention.
Heart symptoms
Chest pain, irregular heartbeat that persists more than a few minutes, palpitations with breathlessness, or near-fainting on standing. Stop fasting and seek urgent care if symptoms are severe or do not resolve within minutes of breaking the fast.
Severe weakness or confusion
Inability to think clearly, slurred speech, difficulty performing simple mental tasks, or feeling “not right” in a way that’s hard to describe. These can indicate hypoglycaemia or electrolyte derangement.
Fainting or near-fainting
Brief lightheadedness on standing is common, especially during longer fasts. Actual loss of consciousness, or feeling so close to it that you have to sit down to avoid falling, is not.
Severe muscle cramps
Beyond mild calf cramps. Severe, persistent, or whole-body cramping suggests significant electrolyte imbalance — usually sodium, potassium, or magnesium.
Vomiting that won’t stop
Especially in someone who is not eating. Risks rapid dehydration and electrolyte loss.
Symptoms of hypoglycaemia in someone on diabetes medication
Sweating, shaking, hunger, anxiety, dizziness, blurred vision, and confusion. Treat immediately with fast-acting carbohydrate (glucose tablets, juice). Do not “wait it out.”
Worsening symptoms despite breaking the fast
If you eat and drink and the symptom does not improve within 30–60 minutes, that is itself a warning sign and warrants medical evaluation.
Adaptation Discomfort vs. Real Danger
Many people have mild symptoms in the first weeks of fasting that are simply part of adjusting. The lists below help separate common adjustment effects from warning signs. When in doubt, eat and seek advice.
Usually adaptation (continue, monitor)
- Mild headache during the first 1–2 weeks
- Hunger waves that come and go (typically 15–30 minutes)
- Irritability or low mood, especially mid-afternoon
- Mild fatigue, particularly in the first week
- Trouble sleeping during the first few nights
- Mild lightheadedness on standing quickly
- Cold hands and feet during a fast
- Bad breath (“keto breath”) on longer fasts
Not adaptation (stop and assess)
- Headache that becomes severe, sudden, or different from previous headaches
- Hunger replaced by nausea, weakness, or shakiness that does not pass
- Persistent low mood that affects daily function for more than a few weeks
- Fatigue severe enough to affect driving, work, or basic care
- Insomnia that continues for more than 1–2 weeks
- Lightheadedness with visual changes, confusion, or actual loss of consciousness
- Heart palpitations beyond the occasional skipped beat
- Any of the symptoms in the previous section
A useful rule: adaptation symptoms tend to improve week over week. Symptoms that worsen, or new symptoms that appear after weeks of stable fasting, deserve attention.
Refeeding Syndrome: Why Long Fasts Need Care
Refeeding syndrome is a potentially serious complication that can occur when food is reintroduced after a long period of very little or no food. It is not a concern with ordinary daily fasting schedules, but becomes a real risk after several days without food (clinical guidelines generally flag more than about 5 days of little or no intake), and a higher risk in anyone who is underweight, malnourished, or has a history of eating disorders.
How it happens: during prolonged fasting, the body’s stores of minerals such as phosphate, potassium, and magnesium become depleted. When food — especially carbohydrate — is reintroduced, insulin rises and moves these minerals into cells quickly, so blood levels can drop sharply. This can cause dangerous heart rhythm problems, breathing difficulties, or seizures.
Practical implications
- In healthy adults, fasts under 48 hours carry very little refeeding risk.
- Fasts of several days should be discussed with a healthcare provider beforehand, and broken with a small first meal and a gradual return to normal eating.
- Fasts beyond 5 days should not be done without medical supervision, and refeeding should follow a medically guided plan.
- Anyone underweight, malnourished, or with prior eating-disordered behaviour is at elevated risk regardless of fast length, and should refeed with medical input.
For more on practical refeeding, see our guide on how to break a fast properly and the electrolyte guide.
How to Lower Your Risk
For healthy adults choosing to fast, the following habits reduce the likelihood of complications.
Start short and progress slowly
Begin with a 12-hour overnight window. Extend by 30–60 minutes every few days or each week until you reach your target. Jumping straight from no fasting to fasts of 24 hours or more makes unpleasant side effects more likely.
Hydrate, with electrolytes on longer fasts
For daily fasting schedules, water and normal salt intake with meals are usually enough. On fasts longer than about 24 hours, the body loses more sodium and water, and low salt intake is a common cause of headaches, fatigue, and lightheadedness. A little salt in water or broth may help. People with high blood pressure, heart failure, or kidney disease should not add salt or electrolyte supplements without talking to their healthcare provider. See the electrolyte guide.
Eat enough during your eating window
Regularly eating too little can cause many of the symptoms on the warning-signs list, as well as muscle loss, missed periods, and binge-restrict cycles. A fasting schedule changes when you eat; it should still leave room to eat enough.
Prioritise protein
Including protein at every meal helps with fullness and with maintaining muscle. Many adults aim for roughly 1.2–1.6 g per kg of body weight per day, and people doing regular strength training sometimes aim higher. People with kidney disease should follow their healthcare provider’s advice.
Don’t fast through illness
Acute infection, fever, or severe stress are not the time to fast. Resume after recovery.
Track how you feel, not just the clock
Ending a fast early because you noticed warning signs is the right call, not a failure. The schedule is there to serve your health, not the other way around.
Tell someone
For fasts longer than 24 hours, especially if you live alone, let someone know your plan and check in with them.
When to Seek Medical Help
Before starting
- If you have any chronic condition
- If you take any prescription medication
- If you are over 65
- If you have a history of eating disorders or are unsure
- If you are planning fasts longer than 48 hours
During fasting
- Any of the immediate-stop symptoms in the section above
- Symptoms of hypoglycaemia in anyone on diabetes medication
- Persistent symptoms that don’t resolve after eating
- Unexplained changes in heart rhythm or blood pressure
Ongoing
- Loss of menstrual periods (women)
- Hair loss, brittle nails, or persistent cold intolerance
- Ongoing weight loss you did not intend, or weight dropping below a healthy range
- Worsening rather than improving blood pressure, glucose, or lipid markers
- Increasing rigidity around eating, fear of food, or distress when fasting is interrupted
The last point is worth emphasising. The line between a fasting routine and disordered eating can blur. If your relationship with food is becoming more anxious rather than less, take it seriously and talk to a healthcare provider, ideally one experienced with eating disorders.
Frequently Asked Questions
Is intermittent fasting safe long-term?
Most clinical trials of moderate schedules such as 16:8 and 5:2 in healthy adults have lasted weeks to about a year and have not found major safety problems when people eat enough protein and calories. Long-term data are limited, especially for more restrictive patterns such as daily OMAD or frequent extended fasts, where there are reasonable concerns about effects on muscle mass, bone health, and reproductive hormones. If you plan to fast long-term, periodic check-ins with your healthcare provider are sensible.
Can fasting cause an eating disorder?
The research is limited. Fasting is not thought to be a common cause of eating disorders in people with no other risk factors, but dieting and food restriction are known risk factors, and a strict fasting structure can reinforce restrictive or binge-restrict patterns in people who are vulnerable. The risk is highest in adolescents and young adults, and in anyone with a personal or family history of disordered eating.
What about gallstones?
Rapid weight loss and long gaps between meals are associated with a higher risk of gallstones in some people. The risk is higher with fast weight loss, in women, and in people with previous gallbladder problems. If you get pain in the upper right side of your abdomen after eating, particularly fatty meals, see a doctor.
Is it safe to exercise while fasting?
Many healthy adults tolerate light to moderate exercise in a fasted state. High-intensity training, long endurance sessions, and heavy strength work often go better after eating, so schedule them inside your eating window when possible. Stop if you feel dizzy, faint, or unwell. See our exercise & fasting guide.
I felt fine, then suddenly felt terrible. What happened?
On longer fasts, a common cause is low sodium and fluid intake, which can bring on headache, fatigue, and brain fog. A cup of salty broth or a pinch of salt in water often helps. Low blood sugar in people on diabetes medication, dehydration, and other health problems can cause similar symptoms. If you don’t feel better soon, end the fast, eat, and seek medical advice if symptoms continue.
Who should I talk to before fasting?
Start with your regular doctor or the clinician who manages any condition you have — they know your medical history and medications, which matters most. Your pharmacist can advise on medication timing. For help planning meals within an eating window, a registered dietitian can be useful.
The Bottom Line on Fasting Safety
For many healthy adults, moderate intermittent fasting is generally well tolerated when done sensibly. For some groups — people who are pregnant or breastfeeding, children and teenagers, people with eating disorders, and those on certain medications — it is not appropriate without medical guidance, or at all. For everyone, knowing the warning signs that mean stop matters more than any potential benefit.
The goal is not to fast as much as possible. If a fast is making you feel unwell, break it and eat. If fasting consistently doesn’t suit you, it is fine to stop — regular, balanced meals are a perfectly healthy way to eat.