Intermittent Fasting Side Effects and Who Should Avoid It

Intermittent fasting side effects are mostly minor. In trials, the ones people reported were hunger, headaches, low energy, feeling cold, constipation, bad temper and trouble concentrating. Large reviews found side effects no more common than on other diets, though the evidence is uncertain. The serious risks apply to specific groups, such as people taking insulin, and to fasts longer than 24 hours.

What intermittent fasting side effects do trials report?

The clearest numbers come from a 6-month trial of 107 women. One group ate about 650 kcal (2,710 kJ) on two days a week, and the other cut calories every day.

Reported in the trial	2-day fasting group	Daily calorie cutting
Complained of hunger	15%	none
Minor physical symptoms: lack of energy, headaches, feeling cold, constipation	8%	none
Minor psychological effects: lack of concentration, bad temper, preoccupation with food	15%	7%
Reported more self-confidence and better mood	32%	46%

Bigger reviews are less clear-cut. A 2024 analysis of 15 trials (1,365 adults with overweight or obesity) covered several kinds of fasting. Fatigue, headaches and drop-out rates were no higher with fasting than with the comparison diets. A 2026 Cochrane review of 22 trials rated the evidence on side effects as very uncertain.

Headaches, low energy and dizziness

Doctors recognise “headache attributed to fasting”. It is a diffuse headache, usually mild to moderate, that starts after 8 or more hours without eating. Eating eases it. The headache is more common in people who already get headaches, and its exact cause is not known.

In the first days of a fast, your kidneys also pass more sodium and water. If a headache eases after a salty drink, low sodium may have been part of it. The guide to electrolytes while fasting covers salt and other electrolytes during a fast.

Low energy was one of the minor symptoms in the trial above. Dizziness was slightly more common with later eating windows in the 2024 analysis, though not by a statistically significant amount. If you take blood-pressure tablets and become dizzy or faint during a fast, end the fast and talk to your doctor.

The fasting troubleshooter covers headaches, dizziness, fatigue, trouble sleeping, irritability and constipation. For each one, it gives likely causes, fixes and when to see a doctor.

Bloating, wind (gas) and other gut upsets

Bloating is usually caused by gas in the gut, and some foods, drinks and conditions such as irritable bowel syndrome (IBS) add to it. On intermittent fasting, look first at the meals in your eating window, since that is when food and drink arrive.

What to check	What to try
Swallowing air while you eat	Chew with your mouth closed
One very large meal to break the fast	Split it into two smaller meals within your window
Fizzy drinks, alcohol, and lots of coffee or tea	Cut back on them
Lots of beans, lentils or cabbage	Eat smaller amounts
“Sugar-free” foods with xylitol, sorbitol or erythritol	Large amounts can cause wind and diarrhoea, so cut down
A large meal late at night	Move your biggest meal earlier in the window

In the EU, foods with more than 10% added sugar alcohols must carry the warning “excessive consumption may produce laxative effects”. Coffee also raises stomach acid and speeds up the bowel, which matters if you drink it on an empty stomach. Constipation was one of the minor symptoms in the 2-day fasting trial.

No trial reviewed for this guide looked at fasting as a bloating treatment. See a doctor if you feel bloated regularly or changing your diet has not helped. Also see one if bloating comes with weight loss you did not intend or blood in your poo. Get urgent medical advice if bloating comes with vomiting, diarrhoea, constipation, a stomach ache, a fever, or a lump in your tummy.

Sleep, mood, hair and periods

Some people find hunger wakes them at night. Short sleep also raises appetite: in a small trial, the fullness hormone leptin fell 18% and the hunger hormone ghrelin rose 28%.

Bad temper and preoccupation with food were among the reported effects. If tracking food or fasting makes you anxious, or breaking a fast often turns into a binge, step back and see a doctor. Free helplines by country are listed at findahelpline.com.

Losing a lot of weight, 9 kg (20 lb) or more, can trigger hair shedding a few months later. The shedding is temporary, and hair usually regains its fullness within 6 to 9 months. Losing weight suddenly or exercising too hard can also stop periods, so see a doctor if you miss 3 periods in a row.

Serious risks and when to stop

• Diabetes medicines. If you take insulin or a sulfonylurea, do not fast without your diabetes team’s advice, and never change diabetes medicines yourself. People with diabetes who fast are advised to break the fast if blood sugar falls below 3.9 mmol/L (70 mg/dL). The same applies if it rises above 16.7 mmol/L (300 mg/dL).
• Medicines taken with food. Take any medicine that needs food with food, even if that ends your fast early.
• Fasts over a day. See a doctor before you go without food for more than 24 hours. After more than 5 days with little or no food, starting to eat again carries a risk of refeeding syndrome, which can be fatal. More than 10 days counts as high risk.
• Emergency signs. Stop fasting and get help straight away for chest pain, fainting or confusion. Do the same if your heart races or beats unevenly, you keep being sick, or your sight changes. For emergency help, call 999 in the UK, 112 in the EU, 911 in the US and Canada, or the local number.

Who should not do intermittent fasting?

Do not start intermittent fasting without your doctor’s advice in these cases:

• you are pregnant or trying for a baby (intentional weight loss in pregnancy is not recommended)
• you are breastfeeding (ask your midwife or doctor)
• you are under 18, since guidance for teenagers puts healthy habits ahead of weight and dieting
• you have an eating disorder now or have had one, or you are underweight (we advise against fasting in either case)
• you have type 1 diabetes, or use insulin or another medicine that can drop blood sugar too far
• your medicines have to be taken with food, or change your blood sugar, blood pressure or fluid levels
• you are frail or older, or getting over an illness or an operation

If you are unsure whether one of these applies, ask a doctor first. The medical disclaimer has the complete list.

Pros and cons of intermittent fasting

Pros	Cons
Weight loss about equal to daily calorie cutting in large reviews	Hunger and minor symptoms for some people
Timing replaces calorie counting	Not safe for several groups without medical advice
Some small trials found lower blood pressure and better insulin sensitivity	Long-term effects unknown, as no trial in the 2026 Cochrane review ran past 12 months

The benefits of intermittent fasting guide rates each benefit by the strength of its evidence. If fasting feels fine but your weight has stalled, read why the scales may not be moving.

If a symptom shows up, pick it in the fasting troubleshooter for the likely cause, the fix and the point at which to stop.