High blood pressure may be more difficult to control in winter, US research suggests.
Exercise can help cut blood pressure
A five-year study found people treated in the summer were on average 8% more likely to see their blood pressure come down to healthy levels.
The US Department of Veterans Affairs team analysed data on 443,632 veterans treated for hypertension.
The study, reported to the American Heart Association, suggests a more active summer lifestyle may be the key.
Lead researcher Dr Ross Fletcher said: "People gain weight in the winter and lose weight in the summer. People tend to exercise more in the summer and less in the winter."
The researchers said it was also possible that people might eat more salty foods in winter. Salt is strongly linked to raised blood pressure.
The study analysed electronic health records from 15 VA hospitals in cities throughout the US.
People with a blood pressure reading of more than 140 mm Hg systolic or more than 90 mm Hg diastolic on three separate days were identified as hypertensive.
The researchers found the same pattern emerged from each hospital they studied, regardless of whether it was based in a warm or cold climate.
Locations ranged from Anchorage, Alaska to San Juan, Puerto Rico.
Dr Fletcher, chief of staff at the VA Medical Center in Washington, said people should be aware of the possibility their blood pressure may be harder to control in the winter - and be more vigilant at this time.
Professor Bryan Williams, a trustee of the Blood Pressure Association, said blood pressure was very variable - even on a minute by minute basis.
However, he said blood pressure levels - and rates of stroke and heart attack - tended to be higher in winter.
This was down to a number of factors, including the fact cold weather constricts the blood vessels.
"Perhaps most important, however, is the impact of the climate on lifestyle," he said.
"We are less likely to exercise and more likely to eat heavier meals in winter, so increasing weight and reducing the benefits of weight reduction and exercise on blood pressure."
Professor Williams added that the level at which doctors intervened to treat blood pressure in the UK was conservative, and so it was unlikely that patients would be over-treated.
High blood pressure typically has no symptoms, but it can lead to major health problems including stroke, heart failure, heart attack and kidney failure.
It is the single most important cause of premature death world-wide.
However, it can be controlled with lifestyle changes and medications.