Aims Heart failure (HF) medication may reduce blood pressure (BP). Low BP is associated with worse outcomes but how this association is modified by HF medication has not been studied. We evaluated the association between BP and outcomes according to HF medication dose in HF with reduced ejection fraction (HFrEF). Methods and results We studied HFrEF patients from the Swedish HF registry (2000-2018). Associations between systolic BP (SBP) and cardiovascular death (CVD) and/or HF hospitalization (HFH) were analysed according to doses of renin-angiotensin system (RAS) inhibitors, beta-blockers and mineralocorticoid receptor antagonists (MRA). Among 42 040 patients (median age 74.0), lower baseline SBP was associated with higher risk of CVD/HFH (adjusted hazard ratio [HR] per 10 mmHg higher SBP: 0.92, 95% confidence interval [CI] 0.92-0.93), which was less high risk under optimized RAS inhibitor and beta-blocker doses (10% decrease in event rates per 10 mmHg SBP increase in untreated patients vs. 7% decrease in patients at maximum dose, both adjusted p < 0.02). Among the 13 761 patients with repeated measurements, 9.9% reported a SBP decrease >10 mmHg when HF medication doses were increased, whereas 24.6% reported a SBP decrease >10 mmHg with stable/decreasing doses. Decreasing SBP was associated with higher risk of CVD/HFH in patients with stable (HR 1.10, 95% CI 1.04-1.17) or decreasing (HR 1.29, 95% CI 1.18-1.42) HF medication dose but not in patients with an increase in doses (HR 0.94, 95% CI 0.86-1.02). Conclusions The association of lower SBP with higher risk of CVD/HFH is attenuated in patients with optimized HF medication. These results suggest that low or declining SBP should not limit HF medication optimization.