EDIT: …Well I messed this thread up. I wasn’t referring to biological half life, but sitting-on-your-shelf half life. (i.e. expiry)
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I mean, you could theoretically calculate the half-life for any substance that you put in your body. It might be difficult, but it’s possible.
Half-life in a pharmaceutical context just means “the amount of time for half of a substance to be eliminated from the body.” Given that you don’t eat rocks, it doesn’t make sense for us to develop a half-life for them. If the body can’t eliminate the mineral, it’s possible the half-life wouldn’t even exist.
Anything you consume which enters your system has a half life for how long it stays in your system. Half lives are the result of how the equations that describe chemical reaction rates are structured.
Pharmaceuticals have half-lives because they are designed to be slowly released into the body over a period of time. This allows the body to slowly absorb the drug and reduces the risk of side effects. Most other things we interact with don’t have half-lives because they are not designed to be slowly released into the body.
A pharmaceutical is a foreign substance to the body. It expends effort to neutralize and remove it through the liver and kidneys. Initially at a higher concentration the removal works faster. Later the chemical is still spread out through your body’s tissues and not as much of it comes into contact with the enzymes and filtering mechanisms per unit of time. Some enzymes may also be depleted and need to be regenerated. So the remaining second half takes longer to process than the first half, and so on.
The pharmaceutical Half life of a chemical is used to determine the dosing schedule for best desired effect and least amount of unwanted side effects.
It’s part of how they determine that you should take 2 ibuprofen every 6 hours vs every 2 hours.
Most other things we put in our bodies, aren’t attached to a dosing schedule per-say so no one has bothered to figure out the half life of the chemicals involved.
The half life of a pharmaceutical is how long it takes the body to eliminate half the drug from the body.
This is important because you want to keep the amount of drug at a therapeutic level, but not have it reach a toxic level.
After 1 half life of the drug has elapsed, there is only half the amount remaining in the body. After 2 half lives, a quarter remains, and so on.
Knowing this, and the therapeutic and toxic levels of the drug, it is possible to calculate an effective dosing regimen.
Many things we interact with actually do have a half life. True, the famous ones are radioactive and [biological](https://en.wikipedia.org/wiki/Biological_half-life) (pharmaceutical) half lives, but any reaction (especially chemical) has a half life. They just aren’t important enough for the average person to identify, and they often have enough things changing them (such as stirring speeding up a chemical reaction) that a specific amount is fairly pointless to keep track of.
The reason for half lives is because most reactions are faster when there are more components to react, and they slow down when there’s less components to fuel the reaction. The simplest way to think about this is as if the components have less surface area interacting with each other as they change into something different, so they stop reacting as often.
…Well I messed this thread up. I wasn’t referring to biological half life, but sitting-on-your-shelf half life.
I’m seeing this after your edit and I don’t see any answers to the question you meant to ask, so I’ll take a stab.
The answer is generally the same though: almost everything *will* “degrade” and lose some of its properties over time, due to any number of factors (erosion from air exposure, chemical breakdown of the components, digestion by microorganisms, etc.). It’s probably more relevant for pharmaceuticals because we need to know how effective that substance is going to be with much higher accuracy than other things. It’s not a super big deal if your spinach has only half the iron as when you bought it, but it *is* a big deal if your insulin has only half the effect as when you bought it.
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