The untold story of 81 mg aspirin

In the Middle Ages, apothecaries—or pharmacists, in plain English—based their system of weights and measures on barleycorn grains. illustration by C Y Gopinath using Midjourney

We're all used to prescriptions, right? Headache? Take a single 500 mg tablet of Paracetamol. Traveller's diarrhoea? Knock it out with 400 mg capsules of Norfloxacin twice a day for three days. By any chance accidentally exposed to anthrax? The doctor prescribes Doxycycline 100 mg capsule twice a day for 60 days.

Notice something? They're all round numbers. Sometimes, you get a number ending in 5, like when a person has rheumatoid arthritis. Then, it would be 325 mg of sulfasalazine. Note, though, that it's still divisible by five.

If you conclude that figuring out a drug's dosage is an approximate science, you would not be far off the mark. A doctor will usually arrive at the best dosage for you by factoring in your age, height, body mass, drug allergies and other medications you may be on.

This might explain why, after they put a stent into my heart some years ago and prescribed exactly 81 mg of aspirin daily, I sat up.

Aspirin, generally regarded as a miracle drug, is best known as a headache-killer, but also as an antipyretic, meaning it brings down fever. One of its effects that cardiologists exploit is its ability to inhibit blood clotting by preventing platelets from sticking together. It is normal for a person who has undergone any cardiac procedure or surgery to be placed on a lifelong daily regimen of aspirin.

Exactly 81 mg of aspirin.

How did they arrive at such an odd and precise number? Logic says that they must have found 80 mg ineffective and 82 mg deadly. But is that how pharmacists determine dosage, milligram by milligram?

I took the question to the Internet's idiot savant, ChatGPT, throwing in Claude AI and Google's Gemini AI for good measure. One piece of pharmaceutical propaganda that floated out was: "The 81 mg dosage for aspirin as a blood thinner is not an arbitrary number, but rather based on extensive clinical research and evaluation . . . Specifically, the 81 mg dosage was found to provide the optimal balance between efficacy and safety. This dose was enough to inhibit platelet aggregation and prevent clot formation, but it minimised the risk of excessive bleeding compared to higher doses."

Bullshit, I thought.

ChatGPT put on its reading glasses and pronounced that the prescription of 81 mg was based on "pharmacological principles and clinical evidence" and that slight deviations from this might have different effects.

Google's Gemini AI told me to suck it up and not question science because the choice of 81 mg was based on "a large amount of research data".

So I shut up. But only for a while.

The truth behind 81 mg can be traced back to what the British and French loved to drink when the sun set: a nice, well-aged whiskey. Specifically, it goes back to the grain most popular among the Scots and the Irish in the Middle Ages for brewing whiskey: barleycorn. They were so nuts about barleycorn grains that apothecaries - or pharmacists, in plain English - based their system of weights and measures on it.

The basic unit of weight was a single grain of barleycorn, unsurprisingly called a grain and abbreviated to gr. In modern terms, a grain would be 64.8 gm. In England, 20 grains equalled one scruple (and not a reflection on the moral standards of the doctor or the patient).

The French, never a nation to follow the British meekly, decided that in their country, 24 grains, not 20, would be one scruple.

In both countries, though, three scruples made a dram, eight drams made an ounce and 12 ounces (that's correct, not 16) made a pound.

Quite overwhelmed by their own cleverness, the British decided that the whiskey grain could also be used to define lengths. They lined up three medium-sized barleycorn grains end to end and thought inch might be a nice name for it. Fortunately, the barleycorn inch was more or less equal to the inch as we know it today.

In 1790, French scientists upset the applecart, by coming up with a new measurement protocol, the metric system. The kilogram was defined as the mass of one cubic decimeter of water at four degrees Celsius. Pretty soon, England went metric too.

However, the apothecary system refused to go away, like a stubborn stain. After repeated outcries from physicians, both systems were deemed legal in Britain till 1898, when the barleycorn-based system was kicked out and the metric system became the law of the land.

America, too, allowed the two systems to run in parallel, though even as late as the turn of the century, only six per cent of American prescriptions were written in the metric system.

In the thick of this merry dance of numbers and units, three medicines have been prescribed since the Middle Ages, always in grains: aspirin, codeine and morphine. The standard adult aspirin dose was five grains. Its metric equivalent, 324 mg, rounded off to 325, remains the prescribed dose for analgesia today. Low-dose aspirin was a quarter of the standard dose, or 1.25 grains, and has remained unchanged until now.

Do the math. A quarter of 324 mg is 81 mg.

You can reach C Y Gopinath at cygopi@gmail.com
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