In 2018, researchers in New Zealand asked people with type 2 diabetes to eat very little on two days a week for 12 weeks. Their glucose-lowering medicines were reduced, and they were taught what to watch for.
Even so, fasting roughly doubled the rate of low blood sugar episodes (Corley et al., Diabetic Medicine, 2018).
That's not a reason to panic — the researchers also found fewer lows than they had feared, thanks to planning. It's a reason to plan. When you fast, your medicines don't know you've changed the rules. Here are the questions worth asking first.
And one rule that runs through all of them: never stop or change your medication without talking to your doctor.
What the evidence shows — question by question
1. "Could this medicine make my blood sugar drop too low if I don't eat?"
This is the big one for insulin and sulfonylureas (tablets such as gliclazide that make the body release more insulin). They keep lowering blood sugar whether or not food arrives. A review of fasting studies in type 2 diabetes found a higher risk of lows in people treated with insulin or sulfonylureas (Rajpal & Ismail-Beigi, Diabetes, Obesity and Metabolism, 2020). During Ramadan, a review of 68 studies found more lows with sulfonylureas than with other diabetes medicines (Abdelrahim et al., Frontiers in Endocrinology, 2021).
2. "Does this medicine need extra care if I'm eating or drinking less?"
SGLT2 inhibitors — medicines whose names end in "-flozin" — can cause diabetic ketoacidosis, a dangerous build-up of acids in the blood. The UK medicines regulator lists "conditions leading to restricted food intake or severe dehydration" among the risk factors. The surprise: it can happen even when blood sugar is near normal, so the usual glucose check may not raise the alarm (MHRA Drug Safety Update, 2016). Warning signs include feeling or being sick, stomach pain, fast deep breathing, sleepiness and a sweet smell on the breath.
In studies of people taking these medicines during Ramadan, a 2026 analysis of 12 studies and 3,625 people found fewer lows, but more dizziness on standing and more low blood pressure (Kamrul-Hasan et al., World Journal of Diabetes, 2026).
Other medicines care about how much fluid you have on board. NHS Scotland's "sick day" guidance explains that water tablets (diuretics) can make dehydration more likely; that some blood-pressure medicines (ACE inhibitors and ARBs) and anti-inflammatory painkillers like ibuprofen may affect the kidneys when you're dehydrated; and that dehydration raises the risk of a rare but serious side effect of metformin. Those rules were written for illness with vomiting, diarrhoea or fever — not for fasting. But they show which medicines are sensitive to fluid.
3. "I take — or used to take — lithium. Is it safe to cut fluids?"
Lithium deserves its own question. It can affect how the kidneys hold on to water in up to 20–40% of people taking it, and in some the effect lasts after stopping. One published case described a woman, off lithium for eight years, whose blood salt rose dangerously when her fluids were restricted (Stone, Journal of the American Board of Family Practice, 1999). The NHS lithium page lists "not eating and drinking much" as a reason to seek urgent advice.
4. "Should this be taken with food — and can the timing move?"
Some medicines are meant to go down with a meal: the NHS suggests taking ibuprofen ideally with or after food, and most people take metformin with food. For others, missing a dose is the risk itself — an NHS guideline on Ramadan names epilepsy and hydrocortisone for adrenal problems as examples. Whether a dose can move into your eating window is a question for your doctor or pharmacist, not for guesswork.
5. "What signs should make me stop and call you?"
Ask for your own list, specific to your medicines. It's the most useful thing you can leave with.
Why the evidence is thin
Most of what we know about medicines and fasting comes from two places: diabetes, and Ramadan, where people fast by day and eat at night. Diet-style fasts in people on blood-pressure tablets, water tablets or mood stabilisers have barely been studied. I searched for a systematic review of blood-pressure medicines during fasting and couldn't find one.
The studies that do exist are small or mixed. The New Zealand trial analysed 37 people. The SGLT2 reviews combine studies that were run very differently. And the regulator's ketoacidosis warning came from reports collected after the medicines were on the market, not from fasting trials.
What you'll hear online
"Fasting got people off their medication." There's a real study behind claims like this. In a Dutch trial, 100 people with type 2 diabetes treated with metformin alone or diet added a five-day fasting-mimicking diet each month, on top of their usual care. After a year, they needed less glucose-lowering medicine (van den Burg et al., Diabetologia, 2024). The trial was co-funded by the company that makes the diet. Notice the details: simple treatment to start with, and every change made by their own doctors under supervision. That's the opposite of stopping tablets on your own.
Before trusting any claim about fasting and medicines, ask five quick questions:
Five questions before you believe a fasting claim
- Was this seen in people, or in mice?
- How many people, and for how long?
- Were they on the same medicines as me?
- Did they measure a real outcome, or a number in the blood?
- Does the person telling me this sell something related? (Not an accusation — just useful to know.)
What would settle it
Trials of fasting in people taking the everyday medicines millions rely on — for blood pressure, heart failure, mood — with careful safety monitoring. Right now, most of that knowledge is borrowed from illness rules and Ramadan studies. It's a gap worth funding.
What you can do
Before you try any fast, write down every medicine you take, including anything you've stopped (lithium especially), and show the list to your doctor or pharmacist. Ask one question about each: what does fasting do to this?
Your doctor will know what applies to you. The conversation takes ten minutes. The plan it gives you lasts the whole fast.
Curiosity first.
— Dr. Brugal
Where to read more
For a safety topic, the best voices are the official sources:
- MHRA Drug Safety Update — SGLT2 inhibitors: updated advice on the risk of diabetic ketoacidosis (2016). gov.uk
- NHS Scotland — Sick Day Rules (which medicines are sensitive to dehydration). therapeutics.scot.nhs.uk
- NHS medicine pages — lithium, metformin, ibuprofen
- IDF-DAR — Diabetes and Ramadan: Practical Guidelines 2021. IDF page
Related in this series: Fasting in Ramadan with diabetes: a practical safety guide (coming soon) · Who should not fast — and the warning signs to stop (coming soon) · Dry fasting: the trend with almost no evidence · Why scientists still can't agree on fasting (coming soon).
References
Studies retrieved via PubMed; official web pages accessed 19 October 2026.
- Corley BT, Carroll RW, Hall RM, et al. Intermittent fasting in type 2 diabetes mellitus and the risk of hypoglycaemia: a randomized controlled trial. Diabetic Medicine. 2018;35(5):588-594. Randomised trial, 41 people randomised (37 analysed), 12 weeks. doi:10.1111/dme.13595 (PMID 29405359)
- Rajpal A, Ismail-Beigi F. Intermittent fasting and 'metabolic switch': effects on metabolic syndrome, prediabetes and type 2 diabetes. Diabetes, Obesity and Metabolism. 2020;22(9):1496-1510. Review of human studies. doi:10.1111/dom.14080 (PMID 32372521)
- Abdelrahim D, Faris ME, Hassanein M, et al. Impact of Ramadan diurnal intermittent fasting on hypoglycemic events in patients with type 2 diabetes: a systematic review of randomized controlled trials and observational studies. Frontiers in Endocrinology. 2021;12:624423. Systematic review of 68 studies, humans. doi:10.3389/fendo.2021.624423 (PMID 33763029)
- Medicines and Healthcare products Regulatory Agency (MHRA). SGLT2 inhibitors: updated advice on the risk of diabetic ketoacidosis. Drug Safety Update, 18 April 2016. Regulatory safety advice. gov.uk
- Kamrul-Hasan ABM, Shaikh S, Ashraf H, et al. Safety and efficacy of sodium-glucose cotransporter-2 inhibitors in adults with type 2 diabetes fasting during Ramadan: a meta-analysis. World Journal of Diabetes. 2026;17(2):115495. Meta-analysis of 12 studies, 3,625 people. doi:10.4239/wjd.v17.i2.115495 (PMID 41696110)
- NHS Scotland, Effective Prescribing and Therapeutics. Sick Day Rules. Patient and prescriber guidance (undated web page). therapeutics.scot.nhs.uk
- Stone KA. Lithium-induced nephrogenic diabetes insipidus. Journal of the American Board of Family Practice. 1999;12(1):43-7. Case report and review. doi:10.3122/15572625-12-1-43 (PMID 10050642)
- NHS. How and when to take lithium (page last reviewed 9 August 2023). nhs.uk
- NHS. Ibuprofen for adults (last reviewed 27 August 2025) and How and when to take metformin (last reviewed 5 August 2026). nhs.uk/ibuprofen · nhs.uk/metformin
- NHS Greater Glasgow & Clyde Managed Clinical Network for Diabetes. Guidelines for the Management of Diabetes Mellitus during Ramadan. Version 6, approved November 2024. Clinical guideline. PDF
- van den Burg EL, Schoonakker MP, van Peet PG, et al. Integration of a fasting-mimicking diet programme in primary care for type 2 diabetes reduces the need for medication and improves glycaemic control: a 12-month randomised controlled trial. Diabetologia. 2024;67(7):1245-1259. Randomised trial, 100 people, 12 months. doi:10.1007/s00125-024-06137-0 (PMID 38546821)
Awareness content — not medical advice. Always talk to your doctor or pharmacist before making changes.