Discontinuation of antihypertensive therapy is a major issue, contributing to poor control of hypertension. Large numbers of patients discontinue their antihypertensive therapy for various reasons. There is some evidence that the choice of antihypertensive may be a factor.

The aim of this study was to compare the influence of antihypertensive drug class on risk of discontinuation using a random-effects model. The population studied included all patients resident in the Eastern Regional Health Authority area of Ireland aged over 35 and eligible for free prescriptions under the General Medical Services scheme who started a new course of antihypertensive monotherapy between August 2000 and July 2001. This population consisted of 14,979 patients, for whom monthly prescription data were available. Patients were selected on the basis that they had received no prescription for the antihypertensive drug considered in the previous 12 months. Drug classes included ACE inhibitors, ß blockers, calcium channel blockers, angiotensin-II antagonists and diuretics. Discontinuation was defined as a period of 3 months or more without receiving any prescription for the drug. Patients were followed until first discontinuation of the drug or until December 2002 or withdrawal from the scheme. The probability of discontinuation was modelled in terms of time period and covariates - sex, age, use of a different antihypertensive in the previous 12 months, and drug class.

Hazard ratios (HR) for discontinuation in relation to the baseline category (male patient aged 70 taking diuretics who had not taken an antihypertensive in the previous 12 months) were calculated with 95% confidence intervals. Use of a random effects model allowed each patient to have a different baseline risk of discontinuation though the effect of the covariates was assumed the same for all patients.

The random effect for patient was significant (variance estimate 1.387, s.e. 0.214, p<0.0001) indicating that the baseline risk of discontinuation varies between patients. The risk of discontinuation was lower for ATII antagonists (HR=0.50, 95% CI 0.43-0.59), ACE inhibitors (HR=0.56, 95% CI 0.51-0.62), calcium channel blockers (HR=0.67, 95% CI 0.61-0.74) and ß blockers (HR=0.68, 95% CI 0.62-0.75) than for thiazide diuretics. This was after adjusting for the effects of age, sex and previous antihypertensive use. Patients who had previously taken a different antihypertensive were at lower risk of discontinuation than patients new to any antihypertensive (HR=0.70, 95% CI 0.65-0.75). Females were at a lower risk of discontinuation than males (HR=0.80, 95% CI 0.74-0.85) and the hazard of discontinuation was quadratic in age with younger and older patients at higher risk of discontinuation.

The risk of discontinuation was significantly higher for patients starting a new course of diuretics than for patients starting a new course of ß blockers, calcium channel blockers, ACE inhibitors, or ATII antagonists. Individual patient factors also have an influence on risk of discontinuation.