Aspirin dosage

I have a really stupid question, but it’s been bugging me for a while, so damn it, I’m going to ask.

I’m a medical secretary, so part of my job is transcribing dictated patient notes. I have noticed that the most common dosage of aspirin is 81 mg. (Though, oddly enough, when I look up aspirin in my pharmaceutical word book, it doesn’t even list 81 mg as a dosage.)

Can anyone shed a little light on why it’s 81 mg? Why not just 80? Does that 1 mg really make that much of a difference? Why not just do 85 mg?

According to my calculations, 81 mg is almost equal to 1 1/4 grains, which I can only assume is considered some sort of standard dose.

Bayer sells a “low dose” version of its aspirin in 81 mg tablets.

A typical aspirin dose was 325 mg. As it turns out, for things like preventing stroke or heart attack it may be better to use lower doses. So, one quarter of the 325 mg dose is about 81 mg.

Also, IIRC, the 81 mg dosage, in the so-called “enteric coated” form at least is subject to patent rights by the manufacturer. (And, cutting a less expensive generic 325 mg tab into four parts won’t work since that would affect the coating and its properties)