Intermittent fasting side effects: is it safe for you?

Intermittent fasting is generally safe for healthy adults, but there are some intermittent side effects to be aware of. Before your first fasting window, it's worth knowing the common side effects (most are temporary), the groups who should check with a doctor first, and the warning signs that mean you should stop.

Not sure how intermittent fasting works yet? Start with our full guide.

Common side effects (what to expect, and how long it lasts)

Hunger and increased appetite — your body is adjusting to a new meal-timing pattern; this is usually most intense in the first 1–2 weeks and eases as your hunger hormones adapt.

Headaches — linked to lower blood sugar, dehydration, and caffeine timing shifts; most common in the 16–30 hour range of longer fasts and often resolves within 30 minutes of adding water or electrolytes.

Irritability and low mood — tied to dropping blood sugar and the general stress of a new routine; typically improves within the first month as eating patterns become habitual.

Fatigue and difficulty concentrating — common in the adaptation phase as your body shifts fuel sources; usually fades within 2–4 weeks for most people.

Sleep disruption — effects vary: some people sleep worse on fasting days, others sleep better once they stop eating late at night; if it persists past a few weeks, it's worth adjusting your eating window.

Digestive changes (constipation, nausea) — often related to eating less fiber or fluid overall during shorter eating windows; usually resolves by prioritizing fiber and hydration during eating periods.

Who should check with a doctor before trying intermittent fasting?

• Pregnant or breastfeeding — caloric and nutrient needs are elevated during this period, and restricting eating windows can unintentionally reduce total intake.
• History of disordered eating — structured fasting rules can reinforce restrictive patterns for some people, even with milder protocols like 16:8.
• Type 1 diabetes, or type 2 diabetes on insulin or other glucose-lowering medication — skipping meals changes blood sugar timing and raises the risk of hypoglycemia; any fasting plan needs medical supervision and likely medication adjustment.
• Underweight, or a history of being underweight — an additional caloric deficit can affect hormone levels, bone health, and immune function.
• Children and adolescents, and older or frail adults — nutrient and calorie needs for growth, or for maintaining strength and muscle mass, make restrictive eating windows riskier in these groups.
• Anyone on medication that must be taken with food, or with a history of fainting, low blood pressure, or a chronic condition like kidney or liver disease — talk to a clinician before changing meal timing.

Signs to stop and see a doctor 

Stop fasting and seek medical advice if you experience andy of the following:

1. Signs of low blood sugar — shaking, sweating, confusion, or fainting — especially if you take insulin or other glucose-lowering medication
2. Severe or worsening headache, dizziness, or fainting that doesn't resolve with fluids and electrolytes
3. Heart palpitations, chest pain, or unusual shortness of breath
4. Signs of disordered eating returning or intensifying, such as preoccupation with food, guilt around eating, or a strong urge to extend fasts beyond your plan
5. Any symptom that feels severe, doesn't improve after a few weeks, or that you're unsure about — when in doubt, check with a doctor before continuing

Frequently Asked Questions

Is intermittent fasting safe to do long-term?

For most healthy adults, current research hasn't turned up serious long-term risks, and some trials run as long as 18 months without major safety issues. That said, most studies still top out around a year or two, so “long-term” evidence is thinner than for many other dietary approaches — periodic check-ins with a doctor are a reasonable precaution, especially past the first year.

Why do I feel so hungry and irritable when I first start?

This is a normal adjustment response as your body gets used to a new meal-timing pattern and to shifts in blood sugar between meals. It's usually most intense in the first one to two weeks and eases noticeably within a month for most people.

Can I drink coffee, tea, or water during a fast?

Yes — plain water, black coffee, and unsweetened tea don't break a fast in the way that matters for most protocols, and staying hydrated actually helps reduce fasting headaches and dizziness. Adding cream, sugar, or other calories during the fasting window changes that.

Will fasting slow down my metabolism?

Short fasting windows used in most popular protocols haven't been shown to meaningfully slow metabolism in the way that prolonged, severe calorie restriction can. Concerns about metabolic slowdown are more relevant to very low-calorie diets sustained over long periods than to typical time-restricted eating.

How do I know if I should stop and see a doctor?

Stop and check in with a doctor if you notice symptoms of low blood sugar (shaking, sweating, confusion), a headache or dizziness that doesn't ease with water and electrolytes, heart palpitations or chest pain, or any sign that restrictive eating patterns are becoming preoccupying rather than routine.

First published: August 2021

Updated: September 2026