Is Intermittent Fasting Safe? Risks and Advice

Intermittent fasting is safe for most healthy adults when followed sensibly, but it is not suitable for everyone. As interest in structured eating patterns such as the 5:2 and 16:8 methods continues to grow across the UK, understanding who can fast safely, who should seek medical advice first, and what side effects to watch for is essential. This article draws on NHS, NICE and British Dietetic Association guidance to help you make an informed decision about whether intermittent fasting is appropriate for your circumstances.

Is intermittent fasting safe for most people in the UK?

Intermittent fasting is generally safe for healthy adults with no underlying conditions, producing modest improvements in weight and cardiometabolic markers, but it is not superior to standard calorie restriction and lacks long-term safety data.

Intermittent fasting is generally considered safe for healthy adults, though it is not suitable for everyone and individual circumstances always matter. Intermittent fasting refers to structured patterns of eating and fasting, the most common being the 5:2 approach (eating normally for five days and restricting calories significantly on two non-consecutive days) and time-restricted eating such as the 16:8 method (eating within an eight-hour window each day).^[3][4] Neither approach is specifically recommended as a first-line dietary strategy in current NICE guidance on weight management, but both have been studied in the context of weight management and metabolic health.^[1][2]

The available evidence suggests that, for otherwise healthy adults with no underlying medical conditions, intermittent fasting carries a low risk of serious harm when followed sensibly. Systematic reviews, including a 2022 review published in the British Medical Journal, have found that various forms of intermittent fasting produced modest but meaningful reductions in body weight and improvements in cardiometabolic markers, broadly comparable to continuous calorie restriction. The British Dietetic Association (BDA) notes that while intermittent fasting can be effective for some people, it is not superior to other approaches to calorie reduction for most individuals.

It is worth noting that intermittent fasting is not a medically supervised intervention in the way that structured weight-management programmes or licensed medicines are. People considering it as part of a broader weight-management plan should be aware that NHS-supported options, including referral to a tier 2 or tier 3 weight-management service, may offer more comprehensive support. Access to these services depends on local Integrated Care Board (ICB) commissioning and eligibility criteria, so availability varies across England. Bolt Pharmacy is a GPhC-registered UK online pharmacy offering online weight-management consultations, with prescriber review and pharmacist support, for those exploring clinically supervised options alongside lifestyle changes.

For most healthy adults, short-term intermittent fasting is unlikely to cause harm, but longer-term safety data remain limited, and individual responses vary considerably.

Who should avoid intermittent fasting or speak to their GP first?

People with diabetes, eating disorders, pregnancy, or those on insulin, sulfonylureas, or SGLT2 inhibitors should consult their GP before fasting, as serious risks including hypoglycaemia and diabetic ketoacidosis can occur.

Several groups of people should either avoid intermittent fasting entirely or consult their GP before starting, because fasting can worsen certain conditions or interact with medications. This is not an exhaustive list, and anyone with a pre-existing health condition should seek personalised advice.

People who should speak to their GP before attempting intermittent fasting include:

• Those with type 1 or type 2 diabetes, particularly if managed with insulin or sulfonylureas, as fasting can increase the risk of hypoglycaemia and may require medication adjustment. Diabetes UK provides specific guidance on fasting with diabetes.

• People taking SGLT2 inhibitors (such as dapagliflozin or empagliflozin), as fasting or very low carbohydrate intake alongside these medicines can increase the risk of euglycaemic diabetic ketoacidosis (DKA), a serious condition that can occur even when blood glucose appears normal.^[7][8] Anyone on these medicines should discuss fasting with their GP or diabetes team before making dietary changes.

• People taking medicines that must be taken with food, including certain anti-inflammatory drugs (NSAIDs), corticosteroids and some antibiotics.

• Those with a current or past history of eating disorders, including anorexia nervosa, bulimia nervosa or binge-eating disorder, as structured restriction can trigger or worsen disordered eating behaviours. Beat (beateatingdisorders.org.uk) provides support and can help people find specialist services.

• People who are pregnant or breastfeeding, for whom calorie restriction is not recommended and may affect foetal or infant nutrition.

• Children and adolescents, whose nutritional needs during growth and development make calorie restriction inappropriate without specialist oversight.

• Older adults, particularly those at risk of sarcopenia (age-related muscle loss) or malnutrition, where fasting may accelerate muscle wasting.^[16]

• People who are underweight (BMI below 18.5) or experiencing unexplained weight loss, who should seek medical assessment before restricting their diet further.

• People with a history of low blood pressure, kidney disease, liver disease or immune-compromising conditions.

Anyone who experiences dizziness, fainting, severe headaches or confusion during a fasting period should stop immediately and contact their GP or call NHS 111 for urgent advice. If someone experiences severe, sudden or crushing chest pain, particularly with breathlessness, sweating or pain spreading to the arm or jaw, they should call 999 immediately. These symptoms may indicate a serious medical emergency unrelated to fasting and require prompt assessment.

What are the known risks and side effects of fasting?

Common side effects include hunger, headaches and fatigue, which usually resolve within two weeks; more serious risks include hypoglycaemia, dehydration, nutrient deficiencies and disordered eating patterns.

Intermittent fasting carries a range of recognised side effects, most of which are mild and transient, though some warrant medical attention. The most commonly reported effects, particularly in the first one to two weeks, include hunger, irritability, difficulty concentrating, headaches, fatigue and sleep disturbance. These often improve as the body adapts to a new eating pattern.

More specific risks include:

• Hypoglycaemia (low blood sugar): Most relevant in people with diabetes or those on glucose-lowering medicines, but can occasionally occur in others during prolonged fasting windows. The NHS provides guidance on recognising and managing hypoglycaemia symptoms, which include shakiness, sweating, confusion and palpitations.^[6]

• Dehydration: Fasting periods can reduce fluid intake, especially if people associate eating with drinking. Adequate hydration throughout the day is important.

• Nutrient deficiencies: If fasting leads to consistently poor dietary quality during eating windows, deficiencies in iron, calcium, vitamin D and B vitamins may develop over time. The BDA recommends focusing on nutritional quality during eating periods to reduce this risk.

• Muscle loss: Very low calorie intake on fasting days, without adequate protein, may contribute to loss of lean muscle mass, particularly in older adults or those who exercise regularly.

• Digestive symptoms: Some people experience constipation if fibre and fluid intake fall during fasting periods, or reflux if caffeinated drinks are consumed on an empty stomach. Maintaining fibre intake and staying well hydrated can help.

• Disordered eating patterns: Some individuals find that restriction triggers binge eating during non-fasting periods, which can undermine both physical and psychological health.

There is no requirement to report side effects from dietary approaches to the MHRA Yellow Card scheme, which is designed for medicines and medical devices.^[10] However, if intermittent fasting is being undertaken alongside a prescribed medicine and an adverse effect from that medicine is suspected, this should be reported to the prescribing clinician and can also be submitted via the Yellow Card scheme at yellowcard.mhra.gov.uk.

Anyone experiencing persistent or worsening symptoms should contact their GP rather than continuing to fast.

How to start intermittent fasting safely with NHS guidance in mind

Start with a shorter fasting window such as 12 hours overnight, prioritise nutritional quality during eating periods in line with the NHS Eatwell Guide, stay hydrated, and review any prescription medicines with your GP or pharmacist before changing your eating pattern.

Starting intermittent fasting safely involves gradual adjustment, attention to nutritional quality and awareness of personal health circumstances. The NHS does not currently publish a specific intermittent fasting protocol, but its broader dietary guidance, including the NHS Eatwell Guide, which emphasises balanced meals, adequate hydration and regular physical activity, applies equally during eating windows.

Practical steps to begin safely include:

• Choose a sustainable pattern: The 5:2 approach or 16:8 time-restricted eating are the most studied. Starting with a shorter fasting window (for example, 12 hours overnight) and extending gradually is less likely to cause adverse effects than moving straight to a 16 or 18-hour fast.

• Prioritise nutritional quality: During eating windows, focus on meals that include adequate protein (to support muscle maintenance), fibre-rich vegetables, wholegrains and healthy fats, in line with the NHS Eatwell Guide. Avoid using eating windows as an opportunity for high-calorie, low-nutrient foods.

• Stay hydrated: Water, plain herbal teas and black coffee or tea without milk or sugar are generally acceptable during fasting periods. The NHS recommends aiming for six to eight glasses of fluid daily as a general guide, though individual needs vary.^[12]

• Monitor how you feel: Keep a simple diary of energy levels, mood, sleep and any symptoms. If side effects are persistent, worsening or concerning, review the approach with a GP or registered dietitian rather than waiting a fixed period.

• Combine with physical activity thoughtfully: Light to moderate exercise is generally compatible with intermittent fasting, in line with the UK Chief Medical Officers' Physical Activity Guidelines, which recommend at least 150 minutes of moderate activity per week for adults.^[13][14] Intense training sessions are best scheduled during or shortly after eating windows to support recovery.

• Review any medicines: If you take regular prescription medicines, discuss timing with your GP or pharmacist before changing your eating pattern, as some medicines (including NSAIDs and corticosteroids) require food for safe absorption or to reduce gastrointestinal side effects.^[15]

For those whose weight or metabolic health requires more structured support, NHS referral to a weight-management service or a consultation with a clinician about evidence-based options remains the most appropriate route. Eligibility and availability depend on local ICB commissioning.

Frequently Asked Questions