What is the Best Time to Take Aspirin

You’ve been prescribed aspirin once daily. When we see the “once daily” instructions on medications, it is usually by default this medication should be taken in the morning. The morning is a convenient time for medications and you’re likely at home and avoid stumbling with pill bottles throughout your busy day. But when it comes to aspirin, there apparently is a difference depending on what time of the day you take it.

What is Aspirin for and How Does it Work?

Aspirin can be used for many cases. One of the most common reasons for taking aspirin is you may have had a heart attack or stroke in the past and taking a low dose aspirin will help prevent protect your heart. Essentially what is happening is your blood is too “sticky” and taking a blood thinner like aspirin helps to “loosen up” the stickiness of your blood to prevent clots (which ultimately becomes a heart attack if the clot reaches your heart or a stroke if it reaches your brain) from happening.

Morning versus Bedtime Taking Aspirin

A study conducted by Dr. Hermida and fellow investigators compared aspirin taken in the morning versus at bedtime. (1) The interesting result between morning and bedtime dosing of aspirin was that those who took aspirin at bedtime experienced an overall lowering of their blood pressure! The following graph summarizes the results:

[caption id=”attachment_168" align=”aligncenter” width=”528"]

Figure 1: Systolic blood pressure over 24 hours for patients taking aspirin in the morning; solid line is the blood pressure before the start of the experiment and the dotted line is at the end of the experiment. There isn’t much difference between the lines, implying blood pressure overall was similar.[/caption][caption id=”attachment_169" align=”aligncenter” width=”519"]

Figure 2: Systolic blood pressure over 24 hours for patients taking aspirin at bedtime; solid line is the blood pressure before the start of the experiment and the dotted line is at the end of the experiment. Noticed the dotted line is lower than the solid line, implying blood pressure was lower when aspirin was taken at bedtime.[/caption]

The main point from Figures 1 and 2 is that those who took aspirin at bedtime, these patients had an overall lowering of their blood pressure by 6.8 / 4.6 mmHg (systolic/diastolic blood pressure). Compared to those who took aspirin in the morning (see Figure 1), there was no appreciable difference in their blood pressure.

We are still not sure of the exact mechanism to the blood pressure lowering effect with taking aspirin at bedtime. Some have offered the hypothesis that taking aspirin helps with blood pressure by helping with nitric oxide production in our body (nitric oxide opens up our blood vessels and lower blood pressure). There is also another hypothesis that perhaps taking aspirin at bedtime better regulates a feedback cycle called the renin-angiotensin cycle.

Bottom line is that if you are using low dose once daily aspirin, taking it at nighttime will give you the additional blood pressure benefit. A nice little bonus!

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(1) Hermida RC, Ayala DE, Calvo C, López JE. Aspirin administered at bedtime, but not on awakening, has an effect on ambulatory blood pressure in hypertensive patients. J Am Coll Cardiol. 2005;46(6):975–83.

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