Emergency calls to Geneva's 144 medical hotline rose about 60% over four days as last week's heatwave pushed temperatures to 37.
The city's university hospital saw ambulatory emergencies climb 10%, while Lausanne's main hospital reported a 15% rise in emergency visits compared with the same period last year.
Doctors are treating heat-related fainting, dehydration, kidney failure and even food poisoning from broken cold chains, mostly in older patients.
Doctors warn that heat effects can appear up to 72 hours after temperatures drop, so hospitals expect the pressure to continue for several days.
A national estimate of excess deaths from the heatwave is expected in about nine days.
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