Short answer. Between them, four medical sources rule out intermittent fasting in pregnancy and breastfeeding, for children and teenagers, with an eating disorder now or in the past, when underweight or malnourished, and with medicine that must be taken with food. Diabetes is on two of the lists, and in a 12-week trial of 41 adults with type 2 diabetes, two fasting days a week roughly doubled the rate of low blood sugar. If you take regular medicine or have a long-term condition, talk to your doctor before you start.
Who should not do intermittent fasting
Four medical sources publish a list of people who should not do intermittent fasting: Australia’s government-funded healthdirect (reviewed December 2025), Harvard’s Nutrition Source (undated, read 2026-11-07), Harvard Health (July 2022) and the Cleveland Clinic (January 2026). The lists overlap more than they differ.
| Group | Listed by |
|---|---|
| Pregnant or breastfeeding | All four |
| Children and teenagers | healthdirect (a child or teenager), Nutrition Source (an active growth stage, such as adolescence), Cleveland Clinic (children) |
| An eating disorder | healthdirect (now or in the past), Nutrition Source (one that involves unhealthy self-restriction, such as anorexia or bulimia nervosa), Harvard Health |
| Underweight or malnourished | healthdirect (underweight), Cleveland Clinic (malnourished) |
| Medicine that must be taken with food | healthdirect, Nutrition Source, Harvard Health |
| Diabetes | Nutrition Source, Harvard Health |
| Older adults | healthdirect |
| Hypoglycaemia (low blood sugar) | Cleveland Clinic |
| «Certain chronic diseases» | Cleveland Clinic, without detail |
Harvard T.H. Chan School of Public Health adds two notes: most studies of intermittent fasting exclude people with a history of disordered eating, and «We don’t recommend intermittent fasting during pregnancy or for children» (Harvard T.H. Chan School of Public Health, 24 September 2025).
If you are in one of these groups, the question is not which plan, but whether to fast at all, and that one is for your doctor. If none of them applies to you, how to start intermittent fasting has a gentle first plan.
Who should talk to a doctor first
Some people are not ruled out, but should not start alone.
- Diabetes medicines. healthdirect says to talk to your doctor first if you take diabetes medicines. Courtney Peterson, Associate Professor of Nutrition at Harvard T.H. Chan, says people with diabetes may need the amount of their medication adjusted to avoid too low blood sugar (Harvard T.H. Chan). The next section shows why.
- Cancer treatment. healthdirect says to talk to your doctor to see whether intermittent fasting is safe for you (healthdirect).
- Medicines for blood pressure or heart disease. People who take them may be more prone to imbalances of sodium, potassium and other minerals (Harvard Health Publishing, undated, read 2026-11-07).
- Any medicine taken at set times. The Cleveland Clinic: «Talk to your healthcare provider before you make any changes to the times you take your medications» (Cleveland Clinic).
- A long-term condition. The Cleveland Clinic’s «certain chronic diseases» comes without a list, so ask about yours.
Diabetes medicines: the plan comes first
Diabetes is where the risk has been measured. In a 12-week randomised trial of 41 adults with type 2 diabetes treated with metformin and/or diabetes medicines that lower blood sugar (Corley et al., Diabetic Medicine, 2018), two low-calorie fasting days a week roughly doubled the rate of hypoglycaemia (relative risk 2.05, 95% CI 1.17 to 3.52), even though doses were reduced. The authors concluded that in people on these medicines, «fasting of any type increased the rate of hypoglycaemia». Harvard Health puts it more generally: «Skipping meals and severely limiting calories can be dangerous for people with certain conditions, such as diabetes» (Harvard Health, 2019).
The Hull University Teaching Hospitals NHS Trust leaflet on fasting with diabetes during Ramadan (Diabetes and Ramadan, updated October 2025) gives practical rules:
- Glucose may drop too low, particularly if you are unwell or treated with insulin or a sulphonylurea (a type of diabetes tablet).
- The risk is higher in older people and in those taking tablets such as diuretics.
- Insulin doses will need to be changed, so speak to your GP, and insulin must not be stopped.
- A glucose reading under 4 mmol/L should be treated.
- See your diabetes team at least 6 to 8 weeks before fasting begins.
Longer fasts raise the bar
A fast of a day or more adds risks of its own. The Cleveland Clinic warns that the side effects of a 24-hour fast can be extreme (Cleveland Clinic) and advises doing prolonged fasting under medical supervision (Cleveland Clinic, Refeeding syndrome, reviewed June 2022). A 2025 review of refeeding in critically ill patients lists a low BMI and unintentional weight loss among its risk factors (Borriello et al., Nutrients, 2025). How long a fast should be covers each length and its risks.
Bare Fast, the app we make, shows safety notes before your first fast longer than a day, and before every fast longer than three days. They do not replace the conversation with your doctor.
Signs to stop
Stop and speak to your doctor if you feel weak, anxious or irritable, can’t concentrate, or can’t do your usual activities (healthdirect). If you have diabetes, treat a glucose reading under 4 mmol/L the way your care team has told you. If a fast is going badly, end it: ending early costs a few hours, nothing more.
What to take to your doctor
- The plan you have in mind, with its hours: for example 14:10, eating from 9 am to 7 pm.
- Every medicine you take, with the times you take it.
- Whether any dose or time should change on fasting days.
- If you take diabetes medicines: what to do about a low reading, and how often to check.
Common questions
Can I do intermittent fasting with type 2 diabetes?
Not without your diabetes team. Harvard’s Nutrition Source and Harvard Health list diabetes among the conditions for which they advise against it, and healthdirect says to talk to your doctor first if you take diabetes medicines. In the trial above, fasting roughly doubled the rate of low blood sugar in people on these medicines, even with lower doses (Corley 2018). The Hull leaflet asks people with diabetes to see their diabetes team at least 6 to 8 weeks before a fast.
Can I do intermittent fasting while breastfeeding?
No, by all four lists above: pregnancy and breastfeeding are on healthdirect’s, Harvard’s Nutrition Source’s, Harvard Health’s and the Cleveland Clinic’s.
Is intermittent fasting safe for teenagers?
The sources say teenagers should not do it. healthdirect lists children and teenagers, Harvard’s Nutrition Source lists an active growth stage such as adolescence, and Harvard T.H. Chan does not recommend it for children.
Can older adults do intermittent fasting?
healthdirect lists older adults among the people who should not do intermittent fasting. The Hull leaflet adds that for people with diabetes, the risk of low blood sugar is higher in older people. If you are older and want to try a plan, ask your doctor first, especially if you take regular medicine.
This post is not medical advice. Talk to your doctor before you change how you eat, especially if you take medication or have a medical condition.