Scientists found variations in blood pressure over long periods, of months and years, raises the risk of ill-health and is a predictor of early mortality in patients who are hypertensive.
High blood pressure is considered a 'silent killer because it causes no symptoms and, if untreated, results in early stroke, heart attack and death.
Researchers at the Institute of Cardiovascular and Medical Sciences at Glasgow University calculated the variability of blood pressure, over four, five and nine years, in 14,522 hypertensive patients.
Their results showed the magnitude of visit- to-visit blood pressure variation was a strong predictor of mortality, independent of their long-term average blood pressure.
Even those individuals who would be considered well-controlled in terms of blood pressure values at each visit, showed a higher risk if they had wide swings in their blood pressure readings between clinic visits.
The current treatment of high blood pressure is regular blood pressure checks and adjusting treatment to get the blood pressure down to safe levels.
Dr Sandosh Padmanabhan, who led the study, said: "Factors such as stress, seasonal effects and people not taking medication regularly can cause increased blood pressure fluctuations.
"The research has implications for how we best manage hypertension in patients.
"For example, physicians will need to give more consideration to blood pressure variability when monitoring and treating high blood pressure.
"The results of our study also highlight the importance of not only taking blood pressure medicines to reduce blood pressure but also taking them regularly.
"Further studies could help identify specific drugs that could reduce variability."