Decongestants provide relief by narrowing blood vessels to reduce nasal stuffiness, but this can potentially affect other blood vessels, which may increase blood pressure. DayQuil HBP is free of decongestants for people with high blood pressure. Does Dayquil affect blood pressure? One of the medications in Dayquil, phenylephrine, works to clear your sinuses and stuffy nose by tightening your blood vessels. As a side effect, this can lead to raised blood pressure. If you have high blood pressure, let your provider know and discuss if Dayquil is the best option to treat your symptoms.