Who Should Not Do Intermittent Fasting?

Most fasting advice assumes you’ve already decided to start and jumps straight to picking a window. This one runs the other way. Some people shouldn’t fast at all, and others shouldn’t start without a doctor involved. Knowing which group you’re in matters more than any 16:8-versus-14:10 debate.

The short version

Intermittent fasting is fine for many healthy adults, but not for everyone. Skip it if you’re pregnant or breastfeeding, under 18, underweight, or have a history of an eating disorder. If you take medication for diabetes (insulin and sulfonylureas especially) or live with another chronic condition, talk to your doctor before you start, because a fast changes how those medications behave. Being outside these groups does not establish individual safety. Stop and get advice if symptoms or concerns arise.

Skip it entirely if this is you

These aren’t “start slow” situations. They’re “not right now.”

Pregnant or breastfeeding. Growing a baby or feeding one raises your nutritional needs, and a fasting window pulls the other way. Johns Hopkins Medicine lists it among the groups who shouldn’t try intermittent fasting.

Under 18. Children and teens are still growing, and that takes regular, balanced meals. Hopkins puts “children and teens under age 18” first on its list of people who shouldn’t try intermittent fasting.

A history of an eating disorder, or a rocky relationship with food. Fasting sets firm rules about when you can and can’t eat. Hopkins advises people with this history to avoid it “because the strict fasting windows can trigger or worsen restrictive or obsessive food behaviors.” If restriction has ever tipped into something harder for you, this isn’t your tool.

Underweight, or recovering from malnutrition. A BMI under 18.5 is the standard cutoff for underweight, and below it the job is to eat more, not less often.

Diabetes and medication: the one to take seriously

This is where fasting moves from “probably fine” to “check first,” so here are the specifics.

If you take insulin or a sulfonylurea (glipizide, glimepiride, glyburide and the like), fasting can send your blood sugar too low. Hypoglycemia is a medical risk, not a rough patch to power through. The Johns Hopkins Patient Guide to Diabetes names it as “the immediate risk” for people on those drugs, and says some medications may need adjusting along with more glucose monitoring. Mark Mattson, the Hopkins researcher interviewed there, doesn’t recommend intermittent fasting for people with type 1 diabetes, and the guide emphasizes medical monitoring. It should not be treated as a current summary of every diabetes guideline.

A second risk is easier to miss. If you take an SGLT2 inhibitor (empagliflozin, dapagliflozin, canagliflozin), fasting or a very low-carb diet can raise the risk of euglycemic diabetic ketoacidosis, a dangerous build-up of acid in the blood that develops while your glucose reading looks normal or only a little high. A 2021 review in Cureus lists fasting and ketogenic diets among its known triggers. Because the glucose number doesn’t sound the alarm, nausea, vomiting, belly pain or unusual tiredness on one of these drugs needs prompt medical attention.

None of this means fasting and diabetes never mix. The same Hopkins guide says fasting can be safe and effective for people with diabetes when it’s medically monitored. The order is what matters: doctor first, sort out the medication and monitoring, then start.

Talk to a doctor first if you have a chronic condition

Between “skip it” and “go ahead” sits a group that can often fast but shouldn’t improvise. Check with a provider before you begin if you have chronic kidney disease, heart disease, low blood pressure, a history of gallstones, or you’re an older adult at risk of frailty (Hopkins includes “older frail adults” on its avoid list). The same goes for any medication that has to be taken with food, since a long fasting stretch can throw off the timing. Your doctor can weigh fasting against your actual medical picture. An app can’t.

If you’re in the clear, start gently

Cleared to try it? Ease in. You don’t need a 16-hour fast on day one. The Hopkins guide quotes Mattson suggesting people narrow their eating window gradually, month by month. Matching the window to your real schedule and building up slowly beats starting on hard mode and quitting by Thursday. Our first-week guide explains what to review when the schedule changes. A quoted adjustment period is not a reason to tolerate dizziness, marked weakness or recurring symptoms.

When to stop and get checked

Even a clean start can turn a corner. Pause and call someone rather than tough it out if you get:

signs of low blood sugar, like shakiness, sweating or confusion
dizziness that won’t clear when you sit down, or any real faintness
fatigue heavy enough to derail your day
weight dropping faster than you intended
food thoughts turning obsessive

That last one counts as much as the physical signs. If fasting starts running your head, it’s costing more than it gives. And if you have diabetes, any hint of ketoacidosis is a reason to get help right away.

Where a fasting app fits, and where it doesn’t

We build one of these, so here’s what it does. Atomic Fast is a fasting timer: it counts your hours and shows which metabolic phase that elapsed time corresponds to. It doesn’t read your blood sugar, doesn’t know your prescriptions and can’t tell you whether fasting is safe for you. That judgment belongs to you and your doctor. What it helps with comes after you’ve been cleared: keeping the window honest, logging meals by photo, typed description or barcode, and settling on a schedule you’ll keep. Start with the doctor, not the timer.

Frequently asked questions

Can I fast if I have type 2 diabetes? Some people may be able to, with individual clinical guidance; it is not suitable for everyone. If you take insulin or a sulfonylurea, fasting can drop your blood sugar too low, and an SGLT2 inhibitor carries its own ketoacidosis risk. Speak with your doctor, who can decide whether fasting is appropriate and direct any medication or monitoring changes. Do not change doses yourself.

Is it safe to fast while pregnant or breastfeeding? No. It’s the clearest case to skip. Both raise your nutritional needs, and a fasting window works against them.

I had an eating disorder years ago. Can I try 16:8 now? Tread carefully. Fasting puts strict rules around eating, and for people with that history those rules can reawaken old patterns. Talk it through with someone who knows your history before you decide.

I get a little lightheaded a few hours in. Do I have to quit? Get somewhere safe and stop the fast. Dizziness has several possible causes, and recurrent symptoms or fainting need assessment. Do not assume it is harmless because it happens early in a new routine.

Sources

Sources checked: October 11, 2026.

Johns Hopkins Medicine — Intermittent Fasting: What is it, and how does it work? — Identifies groups advised not to try intermittent fasting.
Johns Hopkins Patient Guide to Diabetes — An Overview of Intermittent Fasting — Discusses diabetes medication risks and medical monitoring; older guideline commentary should not substitute for current individual care.
Somagutta MR et al. (2021), Euglycemic Diabetic Ketoacidosis and Sodium-Glucose Cotransporter-2 Inhibitors: A Focused Review of Pathophysiology, Risk Factors, and Triggers. Cureus — Explains reported SGLT2-associated ketoacidosis triggers and why normal-looking glucose can be misleading.

If you’ve been cleared to fast, Atomic Fast is free on the App Store and Google Play.

This article is for general education and isn’t medical advice. Intermittent fasting isn’t right for everyone. Talk with a healthcare professional before starting, especially if you’re pregnant or breastfeeding, take any medication, have a health condition or a history of disordered eating, or are considering fasts longer than 24 hours.