Intermittent Fasting Side Effects (and How to Handle Them)

Quick answer: The most common intermittent fasting side effects are headaches, hunger, irritability, fatigue and bloating. The causes are not as settled as the internet suggests: for fasting headaches the mechanisms most often implicated in the literature are low blood sugar and caffeine withdrawal, and the risk rises the longer you fast. Most of these effects ease as your body adjusts over the first week or two. Two slower ones — hair shedding that shows up months later, and gallstones during fast weight loss — have different causes and different fixes. And a few signs, including fainting, a racing heartbeat, confusion, or severe pain under the right ribs, are not side effects to sit out. This guide says which are which.

Intermittent fasting is well tolerated by most healthy adults in the trials done so far (de Cabo & Mattson, New England Journal of Medicine, 2019). But "tolerated" does not mean "comfortable from day one." Your body needs time to adapt to a new eating pattern, and during that transition, you may experience side effects that range from mildly annoying to genuinely unpleasant.

The good news: most common side effects have something you can do about them, even where the underlying cause is not fully worked out. This guide covers each one, and says plainly where the evidence runs out.

Headaches

How common: Very common during the first few days.

Why it happens: Fasting headache is a recognized entity, and honestly, the cause is not settled. A review in Headache (Torelli et al., 2009) found that it usually looks like a tension-type headache, that the chance of getting one rises with the length of the fast, and that hypoglycemia and caffeine withdrawal are the two mechanisms most often implicated. The authors were explicit that much about it remains unexplained. Dehydration is widely blamed, but that review did not identify it as a cause.

Fluid shifts are still worth knowing about. Glycogen is stored with water — roughly 3 grams of water per gram of glycogen in muscle — so as glycogen falls, that water goes with it, carrying sodium out too.

How to fix it:

• Drink water through the day, and more than you think you need in the first week; the water intake calculator turns your weight and activity into a daily target
• Add a little salt to your water
• Some people find a magnesium supplement helps; glycinate and citrate are the usual forms
• If you normally drink coffee, do not quit it the same week you start fasting. Caffeine withdrawal is one of the mechanisms most often implicated in fasting headache, and a Cochrane review found caffeine adds to the effect of painkillers when the two are taken together

For a complete electrolyte strategy, read our guide on electrolytes during intermittent fasting.

Telling One Fasting Headache from Another

The fix depends on the cause, and the cause usually announces itself through timing and company. None of this is diagnostic, but it beats guessing.

Caffeine withdrawal tracks your old coffee schedule. It arrives mid-morning, roughly when you used to drink, feels dull and throbbing across the front of the head or behind the eyes, and eases within half an hour of caffeine. The fix is the obvious one: black coffee is fine during a fast. If you want to cut caffeine, do it in a different month from the one where you start fasting.

Sodium and fluid loss produces a headache that shrugs off plain water and often brings muscle cramps and mild fog with it. Salted water tends to settle it faster than water alone. The electrolyte calculator sets targets for your weight and fast length rather than leaving you to guess at a pinch.

A blood sugar dip comes with shakiness, sweating or a fluttery heartbeat, usually several hours in rather than on waking. In healthy people this is the least common of the three, because the liver holds glucose in a workable range through a normal fast. If it turns up with more than one symptom, break the fast rather than push through.

Some headaches are not adaptation at all. Stop the fast and get medical help for a sudden severe headache, a headache with vision changes, numbness, confusion or one-sided weakness, or one that will not shift after several hours of fluid and salt. Headaches on every single fasting day beyond the first fortnight point at something other than fasting and are worth a doctor's opinion.

Hunger

How common: Close to universal at the start. Most people report it easing over the first week or two.

Why it happens: Ghrelin, the hunger hormone, appears to track habitual meal times rather than actual need for food. Frecka and Mattes (American Journal of Physiology, 2008) found that peak ghrelin concentrations differed between groups according to when each group habitually ate lunch, and arrived ahead of that meal. If you normally eat at 7 AM, expect to feel hungry at 7 AM whether or not there is food.

Hunger also comes in waves rather than building continuously until you eat. Most people find a wave passes on its own.

How to fix it:

• Drink water or black coffee during hunger peaks
• Stay busy; idle time magnifies hunger awareness
• Remind yourself that the wave will pass
• Ensure your meals during your eating window are high in protein and fiber, which improve next-day satiety

For detailed strategies, check our full guide on managing hunger during fasting.

Fatigue and Low Energy

How common: Common in the first few days.

Why it happens: Your body is transitioning from primarily burning glucose to increasingly using fat and ketones for fuel. This switch is not instantaneous (Cahill, Annual Review of Nutrition, 2006). Think of it as a car switching from gasoline to diesel; there is a brief lag.

The fluid and sodium shifts described above may add to it.

How to fix it:

• Do not go low-sodium while you are fasting. Salt your food and your water.
• Keep activity light during the first few days. Walk instead of doing high-intensity training.
• Sleep 7-9 hours. Your body is doing adaptive work, and sleep supports it.
• Be patient. Fasting does not shut the body down: in a study of healthy lean adults fasting for 84 hours, resting energy expenditure rose rather than fell over the first days, alongside a rise in serum norepinephrine (Zauner et al., American Journal of Clinical Nutrition, 2000).

Irritability and Mood Changes

How common: Reported often in the first few days.

Why it happens: Hunger can affect mood. So can the plain psychological work of breaking a deeply ingrained habit — you are used to eating when you feel like it, and the restriction feels frustrating.

Symptomatic hypoglycemia is uncommon in healthy people on standard IF protocols. The liver maintains blood glucose through glycogenolysis and then gluconeogenesis, which hold levels within a workable range through days of fasting in healthy people (Cahill, Annual Review of Nutrition, 2006). If you take insulin or a sulfonylurea, none of that applies to you — talk to your doctor before fasting.

How to fix it:

• Recognize that irritability usually eases as the pattern becomes habit
• Stay hydrated (dehydration worsens mood)
• Consider starting your fast earlier in the evening rather than pushing through a difficult morning
• If mood changes are severe or persistent, widen your eating window

Digestive Issues

How common: Mild digestive changes are common. Severe issues are rare.

What you might experience:

• Constipation: Less food means less bulk moving through your system. This is normal during the adjustment period.
• Bloating after eating: Your digestive system slows during fasting. Eating a large meal quickly after a fast can overwhelm it temporarily.
• Acid reflux: Some people experience increased stomach acid during fasting hours, though research is mixed on this. If you already have reflux, our guide to fasting with GERD covers what tends to change and what to watch for.

How to fix it:

• Constipation: Increase fiber intake during your eating window, drink more water, and consider adding magnesium citrate (which has a mild laxative effect). NHS guidance puts the target for adults at 30g of fiber a day.
• Bloating: Break your fast with a moderate-sized meal rather than a feast. Chew slowly. Give your digestive system time to ramp up.
• Acid reflux: Avoid lying down right after eating. If it keeps happening, speak to a pharmacist or your doctor rather than self-treating.

Bloating

How common: Common enough to deserve its own section, and it comes in two different forms.

During the fast, the gut is quieter. Between meals it runs the migrating motor complex, the sweeping contractions that clear the small intestine (Deloose et al., Nature Reviews Gastroenterology and Hepatology, 2012). Gas from what is left behind can gather rather than move on. Stomach acid also arrives on habit at your old meal times with nothing to buffer it, which is uncomfortable in a way people describe as bloating. Fizzy water adds gas directly, so switch to still if you are prone to it.

After breaking the fast is the more common complaint, and it is nearly always the first meal rather than the fasting. A large meal arriving fast into a slowed gut distends it. Fat delays stomach emptying, so a big fatty first meal sits. And fermentable carbohydrates — the FODMAP group described by Gibson and Shepherd (Journal of Gastroenterology and Hepatology, 2010) — feed gut bacteria that produce gas quickly. Eating fast after real hunger also means swallowing more air.

The usual culprits in a first meal:

How to fix it:

• Break the fast small. A boiled egg, a handful of nuts, some broth. Wait ten or fifteen minutes, then eat the meal.
• Make the first food easy: eggs, chicken, fish, rice, potatoes, cooked carrots. Save the beans and the broccoli for later in the window.
• Chew, and slow down.
• Move fibre later in the window rather than dropping it. The break-a-fast planner builds that order around your fast length.

Most people find this settles within the first few weeks. Bloating that does not improve, especially alongside changed bowel habits, abdominal pain or weight loss you did not intend, is worth a medical opinion rather than another tweak to your window.

Difficulty Sleeping

How common: Occasional, particularly when eating too close to bedtime or in the first few days.

Why it happens: Two likely factors. First, a large meal late in your window sits badly, and many people sleep worse after one. Second, fasting raises serum norepinephrine (Zauner et al., American Journal of Clinical Nutrition, 2000), which is not a settling influence. Neither of these has been pinned down for 16:8 fasting specifically.

How to fix it:

• Stop eating at least 2-3 hours before bed
• If your eating window ends at 8 PM, aim to sleep by 10:30-11 PM
• Avoid caffeine after 2 PM
• Keep your bedroom cool and dark
• If persistent, shift your eating window earlier

Dizziness and Lightheadedness

How common: Occasional, mostly during the first week.

Why it happens: Sodium is the usual suspect. Insulin makes the kidney hold on to sodium: DeFronzo and colleagues (Journal of Clinical Investigation, 1975) showed urinary sodium excretion falling by roughly half when insulin was infused in healthy men. Fasting drops insulin, so sodium goes the other way — out. Lose enough of it and blood pressure sags, which you notice most on standing up quickly.

How to fix it:

• Salt your water or your food; do not go low-sodium while you are fasting
• Stand up slowly, especially in the morning
• If it persists, consider a sugar-free electrolyte supplement; our electrolyte calculator sets daily sodium, potassium, and magnesium targets for your weight and fast length
• If dizziness is severe or accompanied by fainting, stop fasting and see a doctor

Bad Breath

How common: Moderate. More noticeable with longer fasts (18+ hours).

Why it happens: As fat oxidation increases, the body produces ketone bodies, and acetone among them leaves through the lungs (Cahill, Annual Review of Nutrition, 2006). This can create a metallic or fruity smell. It is a sign the fuel switch is happening, but it is socially inconvenient.

How to fix it:

• Brush your teeth and tongue regularly
• Drink more water
• Chew sugar-free gum
• It tends to lessen with time

Difficulty Concentrating

How common: Brief episodes during days 1-3.

Why it happens: The transition from glucose to fat and ketone metabolism can temporarily affect how you feel. Ketones do serve as brain fuel during fasting (Cahill, Annual Review of Nutrition, 2006). Claims that fasting sharpens thinking rest largely on animal studies, and the human evidence is thin (de Cabo & Mattson, New England Journal of Medicine, 2019) — so treat better focus as a nice surprise if it turns up, not a promise.

How to fix it:

• Coffee helps, and keeping your usual caffeine intake steady avoids adding withdrawal on top
• Do not schedule your most demanding cognitive tasks for the first 2-3 mornings
• Most people find it passes within the first week

Hair Shedding

How common: Uncommon, and it arrives late enough that most people blame the wrong thing.

Why it happens: The usual mechanism is telogen effluvium, where a metabolic stressor pushes a large share of follicles out of their growth phase into rest at the same time. They shed together two to three months later. That lag is the whole problem: people notice the shedding in month three, look at what they are doing in month three, and miss the crash diet in month one. Reviews of diet and hair loss put low calories, low protein, and low iron or zinc among the recognised triggers (Guo and Katta, Dermatology Practical and Conceptual, 2017).

Fasting itself is not the mechanism. What happens inside the eating window is. A short window makes it easy to eat far too little without meaning to, and easy to eat the same three convenient things every day, which is how iron and zinc quietly fall short. Fast weight loss makes it likelier still.

How to fix it:

• Eat enough. A deficit large enough to shed hair is larger than one you need.
• Get protein into every meal rather than into one. The dietary floor of 0.8 g per kg of body weight a day is a floor for people who are not losing weight; the macro calculator sets a target from your own numbers.
• Vary the food. A compressed window is not a reason for a narrower diet.
• Ask your doctor for ferritin and zinc tests if shedding continues. Iron deficiency is common in premenopausal women and is a well-established cause on its own.
• Slow the rate of weight loss if it is fast.

Shedding from a nutritional trigger usually reverses once the trigger goes, over months rather than weeks. Patchy loss, a receding pattern, or shedding that does not settle after you have fixed the food is a different problem — thyroid disease, hormonal change and pattern hair loss all present around the same age — and belongs with a dermatologist rather than in a fasting guide.

Gallstones and Fast Weight Loss

This one is not an adjustment symptom, and it is the reason some people should talk to a doctor before they start rather than after.

Why it happens: The gallbladder stores bile and squeezes it out when a meal, particularly a fatty one, arrives. Take the meals away for long enough and it sits still. Bile that sits can become supersaturated with cholesterol, which is the mechanism behind cholesterol gallstones.

The larger risk is not the fasting window but the speed of the weight loss it can produce. When body fat breaks down quickly, the liver pushes more cholesterol into bile than the bile salts can keep dissolved. This is well documented in very low calorie dieting: Liddle and colleagues found gallstones forming in a quarter of patients on a roughly 500-calorie-a-day reduction diet over the study period (Archives of Internal Medicine, 1989). The method does not matter much. The rate does.

Who should speak to a doctor before starting:

• Anyone with known gallstones, and anyone who has had a gallbladder attack
• Women, particularly after pregnancy, and anyone taking oestrogen-containing contraception
• People over 40, people with obesity, and anyone who has recently lost a lot of weight
• People with diabetes or metabolic syndrome, or a family history of gallstones

If several of those describe you, that is a conversation worth having rather than a box to tick.

What lowers the risk:

• Keep to a daily window rather than repeated multi-day fasts. A daily eating window still triggers the gallbladder to empty; a fast lasting days does not.
• Do not crash-diet inside the window. Very low intake is the pattern the research implicates.
• Include some fat when you break the fast. That is what makes the gallbladder contract.
• Aim for steady loss rather than fast loss. The weight loss calculator shows what a slower rate costs you in time, which is usually less than people fear.

Stop and seek medical attention for severe pain in the upper right abdomen, especially after eating; pain spreading to the right shoulder or back; nausea and vomiting alongside that pain; fever and chills with abdominal pain; or yellowing of the skin or eyes. A stone blocking a bile duct is an emergency, not something to wait out.

Fasting does not dissolve stones you already have, and no eating schedule is a treatment for gallbladder disease.

When Side Effects Signal a Problem

The side effects above are normal adjustment symptoms. But some signs indicate you should stop fasting and consult a healthcare provider:

• Fainting or near-fainting that does not resolve with salt and water
• Heart palpitations that persist or recur
• Severe, unrelenting headaches despite adequate hydration and electrolytes
• Extreme anxiety or panic attacks that were not present before fasting
• Significant hair loss (may indicate severe caloric restriction or nutritional deficiency)
• Missed menstrual periods in women (may indicate hypothalamic disruption from excessive caloric restriction)
• Severe pain under the right ribs, particularly after eating, or with fever or jaundice — see the gallstone section above

These are not typical IF side effects. They may indicate an underlying issue or that your fasting protocol is too aggressive for your body.

One more belongs here as a caution rather than a red flag. Intermittent fasting and acne is often claimed as a benefit, and there is a plausible mechanism, since fasting lowers insulin and IGF-1. But the clinical evidence specifically on fasting and acne is limited, and the before-and-afters promise far more than it supports.

How Long Do Side Effects Last?

There is no trial that maps this out, so treat the table below as the pattern our users describe rather than as measured data. Your own timeline may differ:

If any side effect persists beyond 2 weeks despite following the fixes above, reconsider your protocol. You may need a wider eating window, better food quality, or medical guidance.

Know what a normal first week of intermittent fasting looks like to calibrate your expectations.

How Fasted Helps

When you are experiencing a side effect, context matters. Fasted shows you where you are in your fasting window, so you can connect how you feel to how long you have gone without food. If a headache turns up at the same hour every time, that is worth knowing — and worth acting on, whether that means salt, coffee, or a shorter window. Tracking your fasts over time lets you look back and see whether side effects settled as the pattern became habit. Weight tracking on the Pro tier, and the daily protein and fibre entry, give you a rough check that you are eating enough during your window. The app logs no food beyond those two numbers.

Frequently Asked Questions

Are intermittent fasting side effects dangerous?

For healthy adults, the common side effects (hunger, headaches, fatigue) are adjustment symptoms and usually settle. Severe or persistent symptoms are not typical, and are worth taking to a doctor rather than pushing through. The red flags listed above are the ones to stop for.

Can intermittent fasting cause hair loss?

IF itself does not cause hair loss. However, significant caloric restriction (eating too little during your window) can trigger telogen effluvium, a temporary shedding condition. This is a sign you need to eat more during your eating window, not a reason to avoid IF.

Why am I bloated after breaking my fast?

Almost always the first meal rather than the fast. A large meal arriving quickly into a slowed gut distends it; fat delays stomach emptying; and fermentable carbohydrates produce gas fast. Break the fast with something small and easy, wait ten or fifteen minutes, then eat properly. Most people find bloating settles within the first few weeks.

Can intermittent fasting cause gallstones?

The risk that matters is fast weight loss rather than fasting itself. Rapid loss pushes more cholesterol into bile than the bile salts can hold dissolved, and gallstones formed in a quarter of patients on a very low calorie reduction diet in Liddle's study (Archives of Internal Medicine, 1989). A daily eating window still makes the gallbladder empty; repeated multi-day fasts do not. Keep the loss steady, do not crash-diet inside the window, and include some fat when you break the fast.

Is 16:8 safe if I have gallbladder problems?

That is a question for your doctor, not for an article. A daily window is lower risk than an extended fast because the gallbladder still gets stimulated, but lower risk is not the same as safe for you, particularly if you have had a gallbladder attack. Go in with the plan and ask.

Does intermittent fasting affect hormones in women?

Some research suggests that aggressive fasting protocols may affect reproductive hormones in women, particularly if combined with severe caloric restriction and intense exercise. Standard 16:8 intermittent fasting has not been shown to disrupt menstrual cycles in well-nourished women. If you notice cycle changes, widen your eating window and ensure you are eating enough.

Should I stop fasting if I get headaches?

Not immediately. Try water, salt, and keeping your usual caffeine intake steady before quitting — caffeine withdrawal is one of the mechanisms most often implicated in fasting headache (Torelli et al., Headache, 2009). The chance of a headache also rises with the length of the fast, so a shorter window is worth trying. If headaches persist despite all of this, consult a healthcare provider.

Do side effects come back if I take a break from fasting?

Mild side effects may briefly reappear if you stop fasting for an extended period (several weeks) and then restart. Most people report that the second run-in is easier than the first, though we know of no study that has measured this.

What to Read Next

• Electrolytes During Intermittent Fasting: What You Need
• Managing Hunger During Intermittent Fasting
• Your First Week of Intermittent Fasting: What to Expect