People taking antidepressants have been warned to be extra careful during the heatwave.
Health experts say some antidepressant drugs, as well as some antipsychotic medications used to treat conditions such as bipolar disorder and schizophrenia, may prevent the body from regulating its temperature properly.
Dr Wendy Burn, the former president of the Royal College of Psychiatrists, told the BBC that people shouldn't stop taking their medication immediately but that they should seek advice if they are struggling with the warm weather.
Watch: Drought declared in parts of England as heatwave continues
She said that both types of drugs "can cause people's skin to be more sensitive to sunlight and lead to skin reactions".
She said those taking the drugs needed to stay out of direct sunlight and use high factor sunscreen.
Dr Burn advised them to drink plenty of water and avoid strenuous physical activity.
"Those struggling with the side effects should not come off their medication without consulting their doctor or specialist," she said.
Certain types of antipsychotic and antidepressant drugs can cause people to sweat excessively, not realise they are thirsty and make their skin more sensitive to the sun, health experts say.
Dr Laurence Wainwright, from the University of Oxford's psychiatry department, told the BBC there is "evidence to suggest a link between tricyclic antidepressants, antipsychotics and heat-related illnesses".
While still licensed for use in the UK, tricyclic antidepressants are no longer prescribed as often as selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs), which increase the level of serotonin in the brain.
Tricyclic antidepressants block the reabsorption of neurotransmitters serotonin and norepinephrine.
Dr Wainright told the BBC that medications impact the hypothalamus - the part of the brain that controls body temperature.
He said: "In some cases, the body is not able to regulate temperature effectively. The problems that can stem from that include muscle cramps, fainting, heatstroke, heat rash and heat exhaustion."
One of the known side effects of SSRIs, commonly used in the UK is excessive sweating, which can in turn lead to dehydration.
Most of England and Wales is covered by an amber heat warning from the Met Office, with temperatures of up to 34C forecast for Saturday and Sunday.
The warning means heat-related illnesses including sunburn and heat exhaustion are “likely” among the general population.
On Friday, an official drought was declared in eight areas of England by the National Drought Group (NDG), which comprises representatives from the government, water companies, the Environment Agency (EA) and others.