Britain is currently in the grip of its longest heatwave since 1976, prompting widespread advice on hydration and sunscreen.
But the MHRA is warning of a risk that is slipping under the radar, as extreme heat could be compromising how medicines and medical devices work.
In a recent campaign, the regulator warned that warmer temperatures, holiday travel and altered daily patterns can all influence how treatments function – and sometimes in unexpected ways.
The campaign draws attention to several overlooked drug interactions that become more prevalent during the summer.
As social activities ramp up in hot weather, the MHRA warns that people are more likely to drink while on antibiotics, without realising this could affect how they feel the following day.
Grapefruit juice, whether consumed at brunch or mixed into cocktails and mocktails, may also alter how the body processes particular medications, the MHRA cautioned.
Herbal supplements such as St John’s Wort can also clash with prescription treatments.
Perhaps less obviously, illness following a heavy night of socialising can reduce the effectiveness of oral contraceptives.
Dr Alison Cave, the MHRA’s Chief Safety Officer, said: “Summer often means changes to our daily routines, whether that’s socialising more, travelling, or spending more time outdoors.
“It’s worth remembering that changes to your routine can affect how your medicines and medical devices work.”
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She added that the campaign would help people “stay one step ahead” and enjoy the season with confidence.
Jenn Matthissen, the agency’s Head of Gastrointestinal System, Nutrition, Endocrine and Fertility, said: “These interactions can easily slip under the radar because they’re tied to things we enjoy – drinks with friends, trying something new, or changing routines while on holiday.”
Ms Matthissen said awareness rather than avoidance was the key message, noting that patient information leaflets are readily accessible both in medicine packaging and online through the MHRA website.
The regulator is urging the public to consult pharmacists or healthcare professionals when uncertain about combining medications with alcohol, food, or supplements.
Particular vigilance is advised during holidays, festivals, and late nights, as certain medicines require precise daily timing to remain effective.
Anyone experiencing suspected side effects should speak with their doctor, pharmacist, or nurse and report the issue directly through the MHRA’s Yellow Card scheme.
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