As the UK faces a red weather warning with temperatures approaching 40°C, experts have issued a specific warning for people using the weight-loss drug Mounjaro. According to pharmacist Peter Thnoia of NHS-registered pharmacy PillTime, the extreme heat can render the injections 'less effective' if not stored correctly.
Storage requirements for Mounjaro
Mounjaro must be stored in a refrigerator between 2°C and 8°C before use. Once opened, it must be kept below 30°C. 'That means during a heatwave, protecting your medication from high temperatures becomes especially important,' Thnoia explained.
He advises users to avoid leaving pens in warm rooms, on windowsills, or in direct sunlight, as these conditions can expose the medication to temperatures well above the recommended range. Both unused and in-use pens should be kept in the fridge if staying at home.
Checking fridge temperature
Thnoia also warns against freezing. 'If the medication freezes, it should not be used, even after thawing, so make sure your temperature settings aren't too low and avoid placing the jabs at the back of the fridge or next to the freezer compartment where it's typically colder,' he said.
Traveling with Mounjaro
For those leaving home, Thnoia recommends using a cooling pouch or cool packs, such as the ones the medication was delivered in. 'Simply refreeze them, and they're good to go while you're on the move,' he said. However, he strongly advises against leaving Mounjaro in a car, as even short exposure to high temperatures can reduce effectiveness.
'With the injections being once a week, I would always recommend people stick to their set day and take them when it is convenient, rather than travel with them,' Thnoia added. Small changes to the time of day won't affect the medication's effectiveness.
Dehydration risks for GLP-1 users
Dr Suzanne Wylie, a GP and medical advisor for IQdoctor, highlighted additional health concerns for users of GLP-1 receptor agonists like semaglutide (Ozempic) and Mounjaro during hot weather. '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,' she said.
These medications also slow gastric emptying, which can cause individuals to unintentionally eat and drink less. Combined with increased fluid loss through sweating in hot weather, users may be more vulnerable to dehydration-related illnesses such as heat exhaustion and heatstroke.
Gallstones and other concerns
Dr Wylie noted that dehydration can also contribute to gallstone formation, a rare but reported side effect of some weight-loss medications. To prevent dehydration, she advises drinking water regularly throughout the day, even if not thirsty. Adults should aim for six to eight glasses (1.5 to 2 litres) of water daily, with higher amounts needed during hot weather or increased activity.
