going from a regular alcoho drinker to ) alcohol lowers blood pressure by how much

The short version: the evidence here does not directly answer how much blood pressure drops when you stop drinking alcohol, but the closest study gives a number worth knowing.

The best evidence in this retrieval comes from a 1992 randomized trial that gave heavy-drinking hypertensive patients a doctor's advice to cut back [4]. The advice group roughly halved their reported drinking, from about 60 units a week to about 30. Their standing diastolic blood pressure dropped by about 5 mm Hg more than the control group, with a confidence interval of 2.5 to 7.9 mm Hg [4]. That is a modest but real effect. The study was small (41 people completed) and the blood pressure measurement was standing diastolic only, not sitting or systolic, but it was a randomized design with a control group, which is better than most of what exists.

A second study from 1986 compared alcoholics during withdrawal to the same people after a period of abstinence [7]. During withdrawal, supine blood pressure averaged 147/89. After abstinence, it dropped to 121/79 [7]. That is a large drop, about 26 mm Hg systolic and 10 mm Hg diastolic. But these were heavy drinkers in withdrawal, not regular drinkers cutting back. The withdrawal state itself raises blood pressure through sympathetic nervous system overdrive, so the drop after abstinence partly reflects the resolution of withdrawal, not just the removal of alcohol's pressor effect. You cannot apply that number to a regular drinker who stops.

The mechanism is straightforward. Alcohol raises blood pressure by stimulating the sympathetic nervous system and the renin-angiotensin system. When you remove it, those systems downregulate. The 1992 trial shows that cutting consumption roughly in half produces a 5 mm Hg drop in standing diastolic pressure. Going from regular drinking to zero would likely produce a larger drop, but the evidence here does not give that number.

My call: for a regular drinker who stops completely, an estimated drop of 5 to 10 mm Hg in diastolic pressure is reasonable based on the available evidence, with systolic likely dropping more. Confidence: moderate, because the direct evidence comes from heavy drinkers and from a reduction study, not from a zero-alcohol comparison in moderate drinkers.