What does the half-life mean in terms of if the medicine is still working or causing side effects? And why is it 5.5x to completely rid from your system? Thanks.
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A “half life” is a property of systems that decay. And it’s just that the time it takes for some amount of “medication” (or whatever substance that is decaying) to decrease by half is the same, no matter how big your starting dose is. So for medicines, a half life is how long it takes for your body to break down half of the medicine.
Saying that it takes 5.5HL to rid from body is just saying that once you have like 2% of the original dose left, the amount you’d get from halving the original dose 5 full times and a bit more, you can approximate that as being 0% left.
Ok so the other answers and pretty good but missing some details. When you take a medicine, the amount in your body goes up. Immediately, your body starts to digest and process the medicine usually in your kidneys and liver. The more of the medicine you have in you, the faster your body can process it, which leads to the amount in your body following what’s called “exponential decay”. This is similar to “exponential growth” that you might have heard of in regards to COVID. When we are talking about exponential decay, it is kinda complicated to talk about the exact concentrations at specific times, so instead we talk about half lives. This is the amount of time it takes for half of the medicine or whatever you are talking about to decay. Because the rate of decay slows down more and more as the medicine concentration gets lower, this half life will stay the same amount of time, which is why we talk about it. For example if you start out with 100 percent, and your halflife is 1 hour, after 1 hour you would have 50 percent left, another hour and you are at 25, and so on. For your original question, once you get to 5.5 half lives, you will be at 5 percent left. This is probably why they chose 5.5 half lives as a reference because it means that you have 5 percent remaining, and 5 percent is a common cutoff is science. Also, every medicine will have a different half life, some are removed from the body very quickly (half life of a few minutes) and some very slowly (half life of days to weeks).
This has no connection to if the medicine will be working or not. To understand that, you need to know two more things. The first is that all medicines will have an average concentration when they become effective. This is called the theraputic concentration. Above this, the medicine will have an effect, and below that it won’t. This is just an average of people who have been tested, and it will vary between people. This is the level you would be interested in if you wanted to know when the medicine stopped working after you took it. The second number is the toxic concentration. Every medicine also has a concentration above which it is toxic and will start to harm you. Dosing medicine is about staying between the therapeutic concentration and the toxic concentration, and using exponential decay to understand how the concentration changes over time.
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