As Britain swelters through a heatwave that has pushed thermometers above 34°C in London and the South East, millions of people using weight loss injections such as Ozempic and Mounjaro face a hidden danger: a significantly increased risk of dehydration and heat-related illness.
Temperatures reached 34.8°C on May 26, 2026, with forecasts of 35°C in parts of England and Wales, prompting the UK Health Security Agency (UKHSA) to issue amber heat-health alerts for the South East, Midlands and East of England lasting until May 28. The Met Office described the figures as historic for May. In such conditions, anyone taking GLP-1 receptor agonists — the class of drugs that includes semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro) — needs to take extra care, doctors warn.
How weight loss drugs heighten dehydration risk
The medications work by mimicking natural gut hormones to regulate appetite, slow gastric emptying and reduce hunger and thirst cues. While that helps people lose weight, it also means they may eat and drink less without realising they are doing so. “This is largely because some of the most common side effects include nausea, vomiting, and diarrhoea, all of which can lead to significant fluid loss,” explained Dr Suzanne Wylie, a GP and medical advisor for IQdoctor. “These medications also slow gastric emptying, helping people feel fuller for longer, but this can sometimes mean individuals unintentionally eat and drink less overall.”
The hot weather compounds the problem because the body loses additional fluid through sweating. “These losses need to be replaced,” Dr Wylie said. “If someone is already experiencing medication side effects that reduce fluid intake or increase fluid loss, they may be more vulnerable to dehydration-related illnesses, such as heat exhaustion and, in more serious cases, heatstroke.”
The Medicines and Healthcare products Regulatory Agency (MHRA) has issued warnings about severe dehydration associated with these injections, with some cases requiring hospitalisation. Dr Kais Rona, a bariatric surgeon, told researchers that dehydration is a “serious risk” because reduced hunger can also suppress the thirst drive, leading to rapid dehydration. Some experts also suggest GLP-1 drugs may interfere with the body’s thermoregulation, making it harder to cool down effectively in extreme heat.
Another little-known concern associated with dehydration is gallstones. While uncommon, gallstones have been reported as a side effect of some weight loss medications, and rapid weight loss itself is a known risk factor. “Dehydration may further increase the risk of gallstones, because it can reduce bile flow and cause bile to become more concentrated within the gallbladder,” Dr Wylie explained. Ursodeoxycholic acid (ursodiol) can be prescribed to dissolve gallstones or used prophylactically for those at high risk.
Recognising the symptoms of dehydration
Dehydration occurs when the body loses more fluids than it takes in. Left untreated it can become a serious problem, leading to heat exhaustion and potentially life-threatening heatstroke. According to the NHS, the key symptoms include:
- Feeling thirsty
- Headache and feeling light-headed
- Dark yellow, strong-smelling urine
- Peeing less often than usual
- Feeling dizzy
- Feeling tired
- A dry mouth, lips and tongue
- Sunken eyes
Severe dehydration can also cause confusion, muscle cramps, feeling faint or weakness, a rapid heartbeat, and dry skin that lacks elasticity.
Practical steps to stay hydrated
The biggest mistake people make is relying solely on thirst as a cue to drink water, because thirst often occurs after the body has already begun to dehydrate. Instead, the NHS and Dr Wylie advise drinking regularly throughout the day, even if you do not feel thirsty. Adults should aim for around six to eight glasses of water per day — approximately 1.5 to 2 litres of fluid — and more during hot weather, illness or increased activity.
Dr Wylie recommends keeping a water bottle with you at all times, setting reminders to drink, and eating water-rich foods such as watermelon, cucumbers and oranges. If you struggle with plain water, sugar-free squash can add flavour. It is also important to limit sugary drinks, caffeine and alcohol, as these can all dehydrate you.
The NHS further advises staying in the shade between 11am and 3pm, keeping living spaces cool by closing windows during the day and opening them at night, and using electric fans only if temperatures are below 35°C. Cool showers and applying cool water to the skin can also help.
Dr Wylie stresses that anyone experiencing persistent dizziness, severe fatigue, worsening nausea or symptoms of dehydration should seek medical advice. Pharmacists can recommend oral rehydration solutions. People on multiple medications — including antidepressants, blood pressure drugs and certain psychiatric treatments — should also be aware that these can affect the body’s ability to regulate temperature and sweat.
Extreme heat can affect the efficacy of medications such as insulin, Ozempic and Mounjaro. They should be stored correctly, typically below 25°C or refrigerated, as per packaging instructions.
An unexpected hydrating option: milk
While water is the obvious choice, Dr Natasha Dernando, medical director at Medichecks, says milk can be “more hydrating” than water during a heatwave. “A lot of buildings in the UK aren’t well equipped to deal with extreme heat, so it’s more important than ever to look after your hydration levels throughout periods of hot weather,” she explained. “However, you don’t have to stick to water alone.”
The sugar, protein and fat content in milk slow the rate at which fluid empties from the stomach, helping the body stay hydrated over longer periods. Milk also contains sodium, which allows the body to hold onto water for longer. For those who are vegan or lactose intolerant, plant-based alternatives such as soya milk offer similar benefits because they contain electrolytes that aid rehydration.