High blood pressure is considered to be controlled in just under two in five patients in the UK, research suggests.
Around a third of British adults have high blood pressure, or hypertension, which occurs when the force of blood against artery walls is dangerously elevated.
Left untreated, high blood pressure raises the risk of heart attacks and strokes, however, medication can cut the odds of a life-threatening complication by up to 50%.
To better understand how controlled the nation's blood pressure is, scientists from the University of Oxford analysed nearly half a million people aged 40 to 69.
Results reveal that of those being treated for hypertension, just 38% had the condition under control.
High blood pressure rarely causes symptoms, with most cases being diagnosed during a routine check-up.
Hypertension is treatable via medication and lifestyle choices, like quitting smoking and cutting back on salt.
With many cases said to be "undiagnosed or inadequately treated", the Oxford scientists analysed participants of the UK Biobank study who lived within 40 km (24.8 miles) of 22 assessment centres across England, Wales and Scotland from 2006 to 2010.
The team accounted for factors that can influence a person's blood pressure, such as alcohol intake, weight and physical activity level.
Of the more than 450,000 participants, over half (56%) had high blood pressure, of whom 47% had not been diagnosed.
Nearly a third (27%) of those who knew they had high blood pressure were not taking medication to counteract it.
Of the overall participants, the scientists focused on around 99,000 with high blood pressure, who had been diagnosed with the condition on average seven years earlier.
Just over one in 10 (14%) of these participants took three or more drugs to combat their hypertension.
The results further show slightly less than two in five (38%) of the participants who took any high blood pressure lowering-drug had the condition under control.
Controlled blood pressure was defined as a systolic reading of less than 140 mmHg and a diastolic figure under 90 mmHg.
Systolic blood pressure is the force at which the heart pumps blood around the body, with 140 mmHg considered high and 90 mmHg to 120 mmHg "ideal".
Diastolic blood pressure is defined as the resistance to blood flow in the blood vessels. A high reading would be 90 mmHg or over, while an ideal figure ranges from 60 mmHg to 80 mmHg.
The Oxford results further show just over one in five (21%) of the high blood pressure participants were not treated "properly".
Nearly 4% also had a systolic blood pressure reading of at least 180 mmHg, or a diastolic blood pressure figure of 110 mmHg or above, despite being on medication.
Watch: Children also develop high blood pressure
Blood pressure control was found to be worse in the participants who were male, over 60, excessive drinkers, obese or of Black ethnicity.
Earning less than £18,000 ($25,113) a year, having "low education attainment" or working a manual labour job were also linked to poor blood pressure control.
Perhaps surprisingly, smoking – a risk factor for high blood pressure – was associated with a 24% higher rate of hypertension control. The scientists put this down to the individuals undergoing intensive treatment to cut their heart disease risk.
An irregular heart rhythm, migraines, anxiety, diabetes and depression were also linked to blood pressure control.
"Having a comorbidity was associated with higher probability of control, possibly due to more frequent interaction with the healthcare system and/or appropriate management of those at greater cardiovascular risk," the scientists wrote in the journal Open Heart.
"More research is needed to understand barriers to [high blood pressure] control, and the mechanisms underlying the association between [it] and co-morbidities not linked with increased [cardiovascular disease] risk."
The scientists stressed they relied on the participants self-reporting their underlying health and prescriptions. Biobank also does not list how well an individual adheres to medication or healthcare appointments.