Why do our bodies build up a tolerance to some medications but not others?
In: Biology
So first we need to talk about a negative feedback loop. A house thermostat is my go-to example. The furnace makes heat *until* the thermostat senses too much heat and turns it off. That’s simple negative feedback. Now a thermostat that can control the AC and heat at the same time can do two directions of negative feedback and can modulate toward a “center” setting. Many parts of the body use a similar system to control things like heart rate, blood presure, body temp, stomach acid, bone growth, and so many more…
Now most of the drugs that create tolerances will be messing with one of these systems. Now a drug doing its job may be a bit like a space heater. You and your doctor think the room needs to be warmer, so you take your space heater drug. But now your body is kicking on the AC and fighting your space heater drugs, so you need more of them (and then the body fights more and the cycle gets worse), and that’s the crux of where tolerance build-up comes from.
Drugs that **don’t** create tolerance over time often affect body systems indirectly, are absorbed in unusual ways, or are specifically meant to not affect body systems at all. A good example of the last category is any antibiotic, antifungal, or antiparasite medication. The intent when creating one of those is to find a substance that is toxic to the invader you want to kill and doesn’t do anything to the human cells at all. So *you* will never build a tolerance to penicillin because penicillin doesn’t normally interact with your cells. (The bacteria-tolerance is another story) Most of these drugs are imperfect and can cause some side effects. You may actually be able to develop tolerance to those side effects if your body has a regulatory system related to them.
Some medications don’t get absorbed into you at all. Digestive meds are a common one. Simethicone and Pepto don’t need to get into your cells or blood to do their work, and there’s no system that can build a tolerance to their effects. I hope that’s helpful.
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