Does the time of day change the effect of acetylsalicylic acid on blood pressure?

This study investigated if the time of day alters acetylsalicylic acid (ASA; Aspirin) effects on blood pressure (BP) in patients with hypertension (high BP). They found that evening ASA had a greater reduction in BP compared to morning ASA. 

Hypertension or high blood pressure (BP) is very common. Managing hypertension is important to prevent organ damage. Lifestyle adjustments and medication are used to manage BP. There are many types of medication with different modes of action. Acetylsalicylic acid (ASA) is commonly used in patients with hypertension. ASA has several effects. It blocks an enzyme called COX 1. This reduces inflammation in the blood vessels. It also prevents blood from clotting (antiplatelet effect) which can prevent strokes or heart attacks.  

The time of drug administration can change its effects. Circadian rhythm (CR) is the term used to describe the cycle of wakefulness and sleep. BP changes depending on the CR. Certain proteins involved in regulating BP are influenced by the CR. ASA is commonly taken in the morning. However, some studies suggest that the antiplatelet effects of ASA are better in the evening. It is not clear which time of day has a better influence on ASA effects in patients with hypertension.

This study included 114 patients with hypertension. All patients were already treated with 75 mg ASA daily. Before the trial, all patients took ASA in the morning. During this study, patients were assigned to take ASA in the morning (58 patients) or in the evening (56 patients). Another group of 61 patients did not take ASA (control group). The main outcome was the change in BP after 3 months. 

Both ASA groups experienced reduced BP levels when measured in the clinic. However, the evening ASA group had a significantly higher drop in BP compared to the morning ASA group. Also, nighttime BP levels were significantly lower in the evening ASA group (by 3.5%) compared to the beginning of the study. In the control group and the morning ASA group, there was no nighttime BP drop. 

24-hour average BP measurements were also significantly lower in the evening ASA group. 

The authors concluded that evening ASA administration was associated with a greater reduction in BP compared to morning administration.

The number of patients in this study was relatively low. The study only lasted 3 months. A larger long-term study is needed. 

If you have any concerns regarding ASA administration, please discuss this with your doctor.