Aspirin lowers blood pressure when it is
taken at bedtime?
Potential health benefits and research finds   2007
Aspirin can reduce blood pressure, most clinical studies find, but the time for the aspirin
administration is critical. Studies have shown that aspirin produces an administration
time-dependent inhibition of angiotensin II. [6]

Aspirin is a potent antioxidative agent that reduces vascular production of superoxide,
prevents angiotensin II-induced hypertension, and induces NO release. Low-dose aspirin
administered at bedtime, but not on awakening, has also been shown to reduce blood
pressure, possibly enhancing the nocturnal trough in NO production. [7]

In 1981, Walter E. reported that long-term aspirin treatment with 1.5 g/day in
post-myocardial infarction patients could lead to a significant increase in systolic and
diastolic blood pressure after 6 months. He suggested that nonsteroidal antirheumatic
drugs could produce hypertension in animals, probably due to inhibition of prostaglandin
synthesis. [5]

Similar observation was found in animal studies. Acetylsalicylic acid (ASA; 0.1-10
mg/kg), indomethacin (IND; 0.1-1.0 mg/kg), and tartrazine (TZ; 0.1-2.0 mg/kg), given
intravenously into guinea pigs induced dose-dependent increases in mean arterial blood
pressure. [2]

Researchers in Germany treated spontaneously hypertensive rats of the Okamoto strain
with blood pressure above 161 mmHg and SHR with blood pressure levels of less than
160 mmHg with oral doses of aspirin (100 mg kg-1) for three days. They found that
aspirin decreased the blood pressure of the rats with blood pressure above 161 mmHg
but increased the blood pressure of the rats with blood pressure below 160 mm Hg. [3]

Nine men undertook treadmill exercise after pretreatment with placebo, aspirin and
indomethacin. Indomethacin caused a greater increase in systolic blood pressure during
exercise than aspirin and a smaller fall in diastolic pressure than either placebo or
aspirin. Compared with placebo both aspirin and indomethacin attenuated to a similar
degree the increase in calf blood flow and the changes in forearm blood flow following
exercise. These results suggest that although aspirin and indomethacin both inhibit
prostaglandin production they have different effects on exercise-induced changes in
blood pressure. [1]

Low-dose aspirin has also been shown to reduce blood pressure when administered
before bedtime, as opposed to upon awakening, in healthy subjects [8] and subjects with
high blood pressure [7] and pregnant women at high risk for preeclampsia. [6]

In a clinical study of pregnant women at risk of developing gestational hypertension,
Spanish researchers found the time-dependent effects of aspirin on blood pressure.
Aspirin apparently asserted no effect on blood pressure on awakening, aspirin
apparently reduced blood pressure significantly 8 hours after awakening and to a greater
extent before bedtime. [4]

In a 3 month-study of 328 untreated patients with grade 1 hypertension, 44.0 +/- 12.6
years of age, Spanish researchers randomly divided the patients into three groups: [a]
non-pharmacological hygienic-dietary recommendations, [b] the same
recommendations and aspirin (100 mg/day) on awakening, and [c] the same
recommendations and aspirin before bedtime. They found a slightly elevated blood
pressure associated with intake of aspirin on awakening, a significant blood pressure
reduction in the patients who received aspirin before bedtime. [4]

In a 3 month-study of 257 patients with mild hypertension aged at 45+/-13, researchers
also found a highly significant blood pressure reduction in patients who received aspirin
at bedtime but not on awakening. [7]

The ability of angiotensin converting enzyme (ACE) inhibitors to lower blood pressure
may in part be due to the formation of vasodilatory prostaglandins. Inhibition of
prostaglandin synthesis with aspirin may therefore theoretically attenuate the
antihypertensive effect of ACE inhibitors. However, research studies have shown that
intake of low-dose aspirin does not interfere the blood pressure-lowering effect of certain
angiotensin-converting enzyme inhibitors (such as enalapril or losartan) [10, 11] and
statins. [9]


[1] Cowley AJ, et al, Effect of aspirin and indomethacin on exercise-induced changes in
blood pressure and limb blood flow in normal volunteers. Cardiovasc Res. 1985
Mar;19(3):177-80. [2] D'Souza SJ, Biggs DF.Aspirin, indomethacin, and tartrazine
increase carotid-sinus-nerve activity and arterial blood pressure in guinea pigs.
Pharmacology. 1987;34(2-3):96-103. [3] Schirner M, Taube C. Different effects of
aspirin on blood pressure of spontaneously hypertensive rats (SHR) with high and
spontaneously low levels of blood pressure. Br J Pharmacol. 1993 Aug;109(4):900-1. [4]
Hermida RC, et al, Time-dependent effects of low-dose aspirin administration on blood
pressure in pregnant women. Hypertension. 1997 Sep;30(3 Pt 2):589-95. [5] Walter E,
et al, Does chronic aspirin treatment increase blood pressure in man? Klin Wochenschr.
1981 Mar 16;59(6):297-9. [6] Hermida RC, et al, Aspirin administered at bedtime, but
not on awakening, has an effect on ambulatory blood pressure in hypertensive patients. J
Am Coll Cardiol. 2005 Sep 20;46(6):975-83. [7] Hermida RC, et al, Differing
administration time-dependent effects of aspirin on blood pressure in dipper and
non-dipper hypertensives. Hypertension. 2005 Oct;46(4):1060-8. Epub 2005 Aug 8. [8]
Hermida RC, et al, Influence of aspirin usage on blood pressure: dose and
administration-time dependencies. Chronobiol Int. 1997 Nov;14(6):619-37. [9] Magen E,
et al, Effects of low-dose aspirin on blood pressure and endothelial function of treated
hypertensive hypercholesterolaemic subjects. J Hum Hypertens. 2005
Sep;19(9):667-73. [10] Zanchetti A, et al, Low-dose aspirin does not interfere with the
blood pressure-lowering effects of antihypertensive therapy. J Hypertens. 2002
May;20(5):1015-22. [11] Nawarskas JJ, et al, Effect of aspirin on blood pressure in
hypertensive patients taking enalapril or losartan. Am J Hypertens. 1999 Aug;12(8 Pt