OBJECTIVES: Poor medication adherence may lead to complications related to uncontrolled hypertension. It is hypothesized that the use of calendar packaging (CP) can increase medication adherence in hypertensive patients. Thus, our objective was to investigate the effect of CP on medication adherence among hypertensive patients in Malaysia. METHODS: This was a randomized controlled trial of CP (intervention group) versus normal pack (control group) for patients using daily dose of amlodipine. Patients were scheduled to refill their prescription every 28 days for 7 months. The primary outcome was adherence to amlodipine medication, assessed by medication possession ratio (MPR), percentage of refill on time and Malay Version Morisky medication adherence scales (MMMAS). Data were analysed using SPSS software version 22. RESULTS: Eight two patients participated. The mean age (SD) was 55.85 (10.25) and 56.55 (10.42) for intervention and control group respectively. Intervention group showed better medication adherence than the control group in MPR (p<0.05), percentage of patient refill on time and MMMAS. Intervention group have a significant improvement in systolic blood pressure (p < 0.05) compared to control group. Multiple linear regression reported an increase in adherence; MPR (β = 0.006, 95% CI 0.001-0.010) and MMMAS (β=0.467, 95% CI 0.137-0.796) after controlling for age and duration of follow up. Intervention also reduced the systolic blood pressure (β = -4.355, 95% CI –8.419 to -0.291), but there was no significant effect on diastolic blood pressure. CONCLUSIONS: CP was associated with substantial and sustained improvements in medication adherence and systolic blood pressure. The association of improved medication adherence and reduced level of systolic blood pressure suggested that CP can lead to meaningful patient reported outcomes.