Why do antidepressants take several months to build up proper levels but only a few days to metabolize out?
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Every time you take the pill your body saves a small surplus up, the rest either gets used or flushed. By keeping constant resupply over months the baselevel in your body builds up. Once you stop taking them, your body used the surplus instead of the pill. This small surplus is about equal to a few days worth of medication. So it uses the stored surplus up in just a few days.
Many antidepressants are SSRIs or selective serotonin reuptake inhibitors. Serotonin is a neurotransmitter that is made in the brain and is disposed of after use by being reabsorbed or “reuptaken”.
A reuptake inhibitor slows down the removal of the serotonin so more sticks around on average, keeping activating parts of the brain more. When you start taking the SSRI it starts working immediately but you haven’t had time to make more serotonin to build up to the desired level. When you stop taking the SSRI the now higher rate of reuptake will clear the excess serotonin away fairly quickly, much faster than it took to build up.
Imagine your brain is a natural environment, like a field. The good chemicals in your brain (the ones that help you feel calm, happy, satisfied, etc) are like a small river flowing through. All around the river grow lots of plants and trees (or, that is to say, hobbies, things you enjoy, etc). Those can grow big and tall next to the river since they have lots of water flowing past.
Now in someone who’s depressed (due to lack of chemicals), their river is barely a trickle, barely enough to support some shrubs and small trees or grasses. What SSRIs (Selective Seratonin Reuptake Inhibitors) do is build a dam in the river. Over time, that small trickle of water adds up and forms a small pond. Now there’s plenty of water, and once the pond is established, trees and bushes can grow along the banks and we’ve got a nice environment going.
But what happens if you tear down that dam? All of the water in the pond rushes away, and you’re left with a large, bare area where it used to be. The little trickle of water is still flowing, but it’s not enough to fill the pond, let alone to keep the large trees alive. So you have to rebuild the dam and let that trickle of water slowly fill the pond back up, which will take a little while, depending on how much of the water flowed out, or leave it gone and let the small bushes and grasses go back to where the creek flows, which will probably take longer.
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