A retrospective observational cohort study of 1,219,732 individuals aged ≥40 years from the United Kingdom (1998–2018) examined the association of antihypertensive treatment with falls and other serious adverse events in patients with dementia (23,510 cases) versus without dementia. New antihypertensive drug prescriptions were given to 17.3% of individuals with dementia and 11.9% without dementia. Treatment was associated with an increased risk of hospitalization or death for fall (aHR 1.15, 95% CI 1.08–1.22), hypotension (aHR 1.83, 95% CI 1.83–1.85) and syncope (aHR 1.34, 95% CI 1.11–1.61), but not fracture (aHR 1.05, 95% CI 1.11–1.61). CI 0.96–1.15) in individuals with dementia. Similar risks were also true for individuals without dementia (fall: aHR 1.07, 95% CI 1.05–1.10). The absolute risk of falling was higher in individuals with dementia (47 per 10,000 per year, 95% CI 26–70) versus without dementia (14 per 10,000 per year, 95% CI 10–18); likewise for hypotension and syncope. Findings were consistent across analyses. Physicians, patients, and caregivers should consider these risks before new antihypertensive therapy, especially in dementia.