A lot of medicine, when taken, makes the body slack off on making its own version of it. For instance, antidepressants. We take them to regulate chemicals in the brain, and they do a good job, but then the brain gets lazy and slows production. This causes the medicine to work less well, and you have to take more to make up for the lazy brain.
This is the best I could do for ELI5 style- neurochemistry isn’t for little dudes lol
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