Intermittent fasting can cause side effects such as tiredness, dizziness, headaches, mood changes, constipation, and menstrual changes [1]. Mild discomfort may improve with a gentler schedule, but strong, repeated, or worrying symptoms are a signal to stop fasting and get medical guidance.
Key takeaways
- Side effects are more likely when the fasting window is too long, meals are too small, hydration is poor, or the routine does not fit your health needs.
- Do not push through fainting, chest pain, severe weakness, confusion, or symptoms that feel unsafe.
- People with diabetes, medication needs, pregnancy, breastfeeding, or a history of eating disorders need clinician guidance before fasting.
- A shorter fasting window is often a better adjustment than trying to "tough it out."
Common side effects
Possible intermittent fasting side effects include:
- tiredness
- dizziness
- headaches
- mood changes or irritability
- constipation
- diarrhea
- menstrual changes
- difficulty managing diabetes or blood sugar in people affected by those conditions [1]
A 2024 review of randomized trials in adults with overweight or obesity found that intermittent fasting was not linked with a higher overall adverse-event risk than control diets, but common adverse events still included fatigue, headache, constipation, dizziness, and diarrhea [2]. That means fasting may be tolerable for many adults, but it is not symptom-free.
Why side effects happen
Side effects often come from the routine around fasting, not just the fasting window itself.
A few common causes include:
- starting with a strict schedule too quickly
- drinking too little water
- eating too little during the eating window
- missing protein, fiber, or balanced meals
- relying on caffeine to get through the fast
- exercising hard without enough food
- fasting while sleep-deprived or highly stressed
Johns Hopkins notes that it can take 2 to 4 weeks for the body to adapt to intermittent fasting [3]. But adaptation should not mean ignoring symptoms that are severe, repeated, or getting worse.
When to shorten the fast
Shorten your fasting window if symptoms are mild but noticeable. For example, move from 16:8 to 14:10, or from 14:10 to 12:12.
This is especially reasonable if you notice:
- afternoon headaches
- irritability before your eating window
- constipation
- low energy that affects daily tasks
- overeating when the fast ends
- trouble concentrating
A fasting schedule should make your routine easier to repeat, not make the rest of your day harder.
When to stop fasting and get help
Stop fasting and seek medical guidance if you experience symptoms that feel unsafe or unusual, including fainting, chest pain, severe weakness, confusion, persistent dizziness, or symptoms related to blood sugar concerns.
You should also avoid starting intermittent fasting without medical guidance if you are pregnant, breastfeeding, have diabetes, take medication that requires food, or have a history of eating disorders. Intermittent fasting is not appropriate for everyone [1].
How to reduce side effects if fasting is appropriate for you
If fasting is medically appropriate and your symptoms are mild, try adjusting the routine before making it stricter.
Practical changes include:
- Start with 12:12 or 14:10 instead of 16:8.
- Drink water regularly.
- Eat balanced meals with protein, fiber-rich carbohydrates, and healthy fats.
- Avoid using the eating window as a reason to undereat.
- Keep intense exercise closer to meals if fasting makes workouts feel harder.
- Stop increasing fasting hours if symptoms appear.
GoFasting can help you log fasting windows, weight, calorie intake, water intake, and steps so you can review patterns while adjusting your routine. Use tracking as feedback, not judgment.
FAQ
Are headaches normal with intermittent fasting?
Headaches can happen with intermittent fasting [1]. They may relate to hydration, caffeine changes, meal timing, or eating too little. If headaches are severe, repeated, or new for you, stop fasting and seek medical advice.
Can intermittent fasting cause constipation?
Yes, constipation is one possible side effect [1]. Drinking water and eating enough fiber during the eating window may help, but ongoing constipation is a reason to adjust the routine.
Should I keep fasting if I feel dizzy?
No. Dizziness is a sign to pause, eat or drink as appropriate, and reassess. Persistent or severe dizziness needs medical guidance.
Bottom line
Intermittent fasting side effects are not a badge of discipline. Mild symptoms may improve with a gentler schedule, better hydration, and more balanced meals, but strong or repeated symptoms mean the plan should change. The safest fasting routine is one your body can tolerate and your life can realistically support.
Medical disclaimer
This article is for general educational purposes only and is not medical advice. If you have a medical condition, take medication, are pregnant or breastfeeding, have diabetes or blood sugar concerns, or have a history of eating disorders, talk with a qualified healthcare professional before trying intermittent fasting.
References
- Mayo Clinic. "Intermittent fasting: What are the benefits?" https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/expert-answers/intermittent-fasting/faq-20441303
- Zhong F, Zhu T, Jin X, et al. "Adverse events associated with intermittent fasting: a systematic review and meta-analysis." Nutrition Journal. 2024;23(1):72 https://link.springer.com/article/10.1186/s12937-024-00975-9
- Johns Hopkins Medicine. "Intermittent Fasting: What Is It, And How Does It Work?" https://www.hopkinsmedicine.org/health/expert-qa/intermittent-fasting-what-is-it-and-how-does-it-work