It has to do with a unit of measurement we don’t currently use for dosing medications, called a grain. The full dose of aspirin was 5 grains, which is about 325mg. The low dose was a quarter of the full dose, so 1.25 grains (which is roughly 81mg). That was the dose that was on formulary, and since it falls within the low-dose range and it was the dose available, it stuck.

Brief overview of aspiring dosing history

For daily prophylactic use, it’s 81mg for all body weights (though the actual definition of low-dose aspirin is anything less than 100mg per day - 81mg is traditionally the dose for historical reasons).

Current Aspirin Guidelines

Largely because there isn’t an increase in efficacy for higher doses of aspirin for prevention of cardiac or stroke events, but there is an increased risk of bleeding events with increasing doses according to several studies.

Effects of Aspirin Dose When Used Alone or in Combination With Clopidogrel in Patients With Acute Coronary Syndromes

Double-dose versus standard-dose clopidogrel and high-dose versus low-dose aspirin in individuals undergoing percutaneous coronary intervention for acute coronary syndromes (CURRENT-OASIS 7): a randomised factorial trial

We colloquially refer to sodium chloride as “salt” and it leads many people to associate the word salt with food. For everyday purposes, that meaning is perfectly fine. However the word “salt” as used in chemistry simply means a neutrally charged compound consisting of positive cation(s) and negative anion(s). Sodium chloride (NaCl, or Na ^+ Cl ^- ) meets this criteria, hence it being called salt.

The same chemical definition fits amphetamine salts and the formulation of lithium used clinically, however that’s the end of their similarity with food items. When using these medications, it isn’t the fact that patients get a “salt” that’s causing a clinical change, but rather that the medications have specific mechanisms of action (e.g. Lithium reduces excitatory stimulation of dopamine and glutamate and up-regulates GABA, though it’s actual mechanism of action is largely unknown).

That doesn’t mean that diet could not, or does not, affect mental health disorders, just that diet’s effect is a completely different topic.

I remember one an IM attending discussing anuric/oliguric patients with ESRD on dialysis. He jokingly called the GI tract “the biggest nephron.”