Intermittent Fasting Side Effects: When to Stop

In 30 seconds

Hunger, headache, dizziness or irritability may appear at first and often ease with time. With diabetes, fasting can also cause low or high blood sugar, dehydration or ketoacidosis; insulin or other diabetes medicines need a clinician-supervised plan. Older adults should talk with a clinician first because evidence is especially limited, and anyone with severe or rapidly worsening symptoms should stop and seek care.

• At the start, trials and reviews report hunger, irritability, trouble concentrating, headache, dizziness, nausea or constipation; these often ease as people adapt.
• In people with diabetes, fasting can cause low or high blood sugar, dehydration and ketoacidosis, so insulin and some other medicines need a clinician-supervised plan.
• Long-term safety is uncertain, and trials mostly enrolled adults aged 60 or younger, so results don't necessarily apply to children or older adults.
• Stop fasting if you feel dizzy, shaky, sweaty or confused, and get emergency help for confusion, unusual sleepiness, a seizure or fainting.

Which side effects appear at first?

Most side effects reported in fasting research are mild and show up early, while you adjust to longer gaps between meals. For practical help, see hydration during intermittent fasting and how to handle hunger.

When people start intermittent fasting, reviews and trials report hunger, irritability, trouble concentrating and mild effects such as headache, dizziness, nausea or constipation; these often ease as people adapt.Source 33Source 44Source 55

Who faces higher risks?

The biggest risks sit with people who have diabetes, especially those on medicines that can push blood sugar too low.

In people with diabetes, low blood sugar is the number one fasting risk; insulin, sulfonylureas and meglitinides (glinides) are the medicines most linked to it, so doses need a plan made with a clinician.Source 11

Cutting insulin too far to avoid low blood sugar can cause high blood sugar and even diabetic ketoacidosis, and SGLT-2 inhibitors and diuretics add a risk of dehydration.Source 11

An NIDDK expert interview for health professionals ranks low blood sugar as the number one fasting risk in diabetes, followed by high blood sugar, ketoacidosis and dehydration. It is one clinician's guidance, not a trial.Source 11

Age matters too, because most fasting research has been done in younger adults. If you are over 60, read intermittent fasting for older adults.

A National Institute on Aging overview says the long-term safety of fasting diets is uncertain and that trials typically recruited adults aged 60 or younger, so the results don't necessarily apply to children or people over 60.Source 22

When should you stop fasting?

Low blood sugar is rare in people without diabetes, but feeling dizzy, shaky, sweaty or confused during a fast is still a reason to stop and eat.

Get emergency help if someone with signs of dehydration becomes confused, sleepier than normal or hard to wake, or if someone with very low blood sugar is not responding normally, has a seizure or becomes unconscious.Source 66Source 77

When to stop the fast and seek care

1. Dizzy, shaky, sweaty or weak

   Stop and break the fast

   If you feel dizzy, shaky, sweaty, confused, very weak or faint, stop the fast and eat or drink something. On insulin or other diabetes medicines, treat it as low blood sugar with something that raises it quickly. Contact a clinician if symptoms don't improve.Source 66Source 77

2. Confusion, fainting or a seizure

   Get emergency help

   Call an emergency number if someone becomes confused, is sleepier than normal or hard to wake, has a seizure, or is not responding normally. These can be signs of severe dehydration or very low blood sugar.Source 66Source 77

3. After a severe episode

   Get advice before fasting again

   If you needed help for low blood sugar or dehydration, talk with your clinician before fasting again. With diabetes, medicines need a plan made with them, and people on insulin should check their glucose more often during the first fasts, until the adjusted dose is confirmed.Source 11Source 66

Who needs a plan before fasting?

Talk to a qualified health professional before fasting if you are pregnant or breastfeeding, under 18, living with or recovering from an eating disorder, have diabetes, take insulin or a medicine affected by meal timing, are an older adult, or are acutely ill. Do not change the dose or timing of any medicine without your clinician. See Blimbi’s safety guide.

Related guides

• Intermittent Fasting With Diabetes: Is It Safe?

  Intermittent fasting with diabetes: plan with a clinician. Learn which medicines raise low blood sugar or dehydration risk during a fast.

• Intermittent Fasting for Older Adults: Is It Safe?

  Intermittent fasting for older adults: the US NIA says there is not enough evidence to recommend fasting diets. What to ask your clinician first.

• Dry Fasting: What It Is and Why It Isn't Safe

  Dry fasting means no food and no fluids, not even water. Why the dehydration risk makes it unsafe, the warning signs to know and what fasting trials allowed instead.

• Electrolytes and Dehydration During Intermittent Fasting

  Do you need electrolytes during intermittent fasting? Not for an ordinary fast, per official sources. Learn the dehydration signs to watch and when to end a fast.

• How to Curb Hunger During Intermittent Fasting

  Hunger during intermittent fasting is expected, and trials do not show that fasting reduces it more than eating less. A gentler window may help.

Frequently asked questions

Can I fast if I take blood-sugar or diabetes medicines?

Not without a clinician-supervised plan. Low blood sugar is the main risk with insulin, sulfonylureas and glinides, and cutting doses too far can push blood sugar too high. Never change medicines on your own.Source 11

Are fasting diets safe for older adults?

Evidence is limited. The National Institute on Aging says there isn't enough evidence to recommend any fasting diet and that trials typically recruited adults aged 60 or younger. Talk with your healthcare provider before starting.Source 22

Sources

1. 1

   Fasting Safely with DiabetesMartin M. Grajower. National Institute of Diabetes and Digestive and Kidney Diseases, August 26, 2020.Official guidance

2. 2

   Calorie restriction and fasting diets: What do we know?National Institute on Aging. National Institute on Aging, August 14, 2018.Official guidance

3. 3

   Effects of Intermittent Fasting on Health, Aging, and DiseaseRafael de Cabo, Mark P. Mattson. Massachusetts Medical Society, December 26, 2019.DOI: 10.1056/NEJMra1905136 · PMID: 31881139Narrative or mechanistic review

4. 4

   Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trialsZhila Semnani-Azad et al. BMJ, June 18, 2025.DOI: 10.1136/bmj-2024-082007 · PMID: 40533200Systematic review and meta-analysis

5. 5

   Effects of timing and eating duration of time restricted eating on metabolic outcomes: systematic review and network meta-analysisYu-En Chen et al. BMJ, January 21, 2026.DOI: 10.1136/bmjmed-2024-001071 · PMID: 41586347Systematic review and meta-analysis

6. 6

   Low blood sugar (hypoglycaemia)NHS. NHS, August 3, 2023.Official guidance

7. 7

   DehydrationNHS. NHS, May 1, 2026.Official guidance