No drug where a 2% difference matters can be administered outside of a monitored hospital setting. There are too many variables that affect the drug uptake far more than 2%, such as body weight (100 pound person will have 3x the drug dosage of a 300 pound person), drug absorption (based on body chemistry, other food eaten, etc.) and the actual intake dosage itself (Eye drops, for example, can have a 500-fold variance in how much actually gets administered). That's why over-the-counter drugs have to have very high therapeutic index. https://en.wikipedia.org/wiki/Therapeutic_index

> There are too many variables that affect the drug uptake far more than 2%, such as (...)

...or in case of tablets that come in 2x/4x the dose and you need to cut them in half/four pieces, the precision of your cut and amount of tablet dust that is lost.

And the distribution of the active ingredient vs filler.

It's a lot of guess work.

IANAD but it probably means some of the drug molecules have been rendered chemically inactive.

For many drugs a 2% difference is probably unnoticeable. Why are Ibuprofen pills 200mg? Because it's a nice round number that happens to work for many people. If I have a headache I often break the 200mg pill and only take half of it. It still does the job fine for me with fewer side effects.

I'm pretty sure for almost all drugs 20% doesn't matter. Think about it. I've never gotten an over the counter or prescription drug that was adjusted for my weight. The closest thing I can think of is painkillers (tylenol, etc) giving different dosages for children.

So if a 110lb wife gets the same dosage as a 200lb husband, how on earth would even a 20% difference in dosage amount make a big difference?

2% can only be done in fantasyland, like in a hospital you and the medicine are both closely weighed and administered.

> I've never gotten an over the counter or prescription drug that was adjusted for my weight.

How do you know that your doctor didn't take your weight into consideration when selecting the dose for a prescription medicine?

OTC drugs are often generally provided at a dose that should be safe for most adults, but may be significantly different in effectiveness based on weight.

Because my dosage has never varied, while my weight has and by more than 20%.

>I wonder for certain drugs how much of a difference 98% potency is vs 100%. Surely that 2% must matter.

Why is this a "surely"? On a scale of 100%, 2% is 2% because it's not hugely significant.

You can get a rough idea of the required precision by checking if the dose is weight dependent. If it isn't, 2% won't matter.

And if 2% does matter, you'd better make sure you can measure both the dose and your weight to better than that accuracy.

Chemical reactions between air, mostly, and to a lesser extend between the drug and the pill substrate itself, I would imagine. I'm sure on such small size and long time scales the pill does simply ablate some of it's material.

Disclaimer: this is conjecture.