Are you taking ASA (aspirin) to lower your blood pressure? If you do, take it before going to bed!

People with pre-hypertension (<140/90 mm Hg) taking 100 mg ASA per day showed statistically significant drops in both systolic and diastolic blood pressure (-5.4/-3.4 mm Hg), but only if they took the medication before going to bed. (1). There was no blood pressure change when ASA was taken in the morning. Pre-hypertension is a risk factor for cardiovascular disease.

The same researchers had earlier found the same pattern in patients with mild hypertension (2). Only ASA taken before going to bed resulted in blood pressure drops (-4.9/-3.5 mm Hg). Blood pressure varied throughout the day, but after evening ASA administration it remained consistently lower all day, compared to pre-treatment values.

It isn't known why taking aspirin is more effective at night. One possible explanation is better absorption by the gastrointestinal tract at night. ASA taken before going to bed may also slow the production of hormones and other substances that may cause clotting, many of which are produced by the body at rest. (1)

This drop in blood pressure at night only holds true for low doses of ASA. The much larger amounts taken for anti-inflammatory effects (500 mg/day or more) may actually increase the blood pressure, even if the medication is taken at night. However, even there it is still better to take ASA before going to bed. Doses of 1300 mg of ASA at night resulted in 37% fewer gastric hemorrhagic lesions, i.e. even large doses of ASA are better tolerated at night. (2)

Sources

1. Aspirin at bedtime lowers blood pressure. HealthDay News May 14, 2008.
2. Hermida RC et al. Administration time-dependent effects of aspirin on blood pressure in untreated hypertensive patients. Hypertension 2003;41:1259-1267.