Can someone Explain how half life of meds works with tapering?

So if my Amisulpride has a half life of 12 hours, does that mean that if I make a reduction that within 24 hours that level of med is gone and reduced to my new dose?

I need to understand this.

Now before I take my next dose I feel I need it as I am feeling a bit spaced out and my head feels weird and concentration is a bit off.

I take the meds and it gets better.

Need to understand this so I know when to drop next dose

After every half time, the quantity will be reduced by 50% of the previous value, not by 50% of the initial value.
Within 24 hours the quantity will be reduced to half 2 times, because 24 hours = 2x the half time.

So at t=0 hours quantity is 100%; at t=12 hours quantity is 100/2 = 50%; at t=24 hours quantity is 50/2 = 25%.

Every time you take another med, you add that to the current quantity. So if you take 100% dose after 24 hours has passed since your last dose, it will be 25% (the current %) + 100% = 125% of the medication is left. Now if you wait another half life (now t=36h) the quantity will be reduced half again: 125/2 = 62.5%.

Instead of % you can put milligrams, so if you took 100mg at t=0h and 100mg at t=24h, the quantity of the medication in your system at t=24h is the same as if you just took 125mg of it for the first time ever.


The above assumes 100% bioavailability (100% of the dose gets into your system).
If the bioavailability of your drug is for example 60%, 60% of the dose gets into your system, and in 1 half-life the amount will be reduced to 30%.
When you take another dose, multiply it by the bioavailability (0.6 in this example), and add the resulting value to the amount that is left from taking the drug previously.

Using the 60% bioavailability as an example, if you take 100mg initially: 100mg*0.6 = 60mg gets into your system. After 1 half life has passed 60mg/2 = 30mg is left in your system. If you now take another dose, 30mg + 60% of 100mg (another dose) = 90mg of drug is in your system.

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I find it hard to understand, but I think this is really helpful.

Will read it again when I am more awake


You can do an experiment on that calulator if you want.

Put 100 in “inital quantity” (100mg dose), 24 in “time” (time passed since last dose), 12 in “half-life” (12h half-life assumed in this example)
Press Calculate, and you get the result that after 24h has passed the “quantity remains” = 25(mg)
Each time after you press calculate, add the “quantity remains” to the current value in the “inital quantity” box. Now “initial quanity” = 100+25, so put 125 in the “initial quantity” box. Press Calculate.

“quantity remains” = ~31(mg). Add another dose (100mg) to that, so you’ll put 131 in the “initial quantity” box. Press Calculate.

“quantity remains” = ~33(mg). Add another dose (100mg) to that, so you’ll put 133 in the “initial quantity” box. Press Calculate.

“quantity remains” = ~33(mg).
As you can see, the quantity of the drug in your system (“quantity remains”) won’t increase any more, it will level out.

If you keep taking 100mg of the drug every day, the amount in your blood keeps fluctuating between 133mg (just after taking a dose) and 33mg (just before taking the next dose).

In this example I used approximation (~ sign), so actually the amount will keep increasing in your system, but the increase will be so small (and will get even smaller after every 24 hours) it can be ignored.

This again assumes 100% bioavailability (100% of the drug enters your blood and nothing gets lost in digestion, etc). If the bioavailability is less, put the actual value - for example 60mg - in the “initial quantity” box. Every time 60mg + “quantity remains” will be put in the “initial quantity” box.


Zombie…if you need to revisit please open a new topic!

Typically, we like to leave inactive threads alone and just start a new thread instead. This is different than how some forums are run, but we do it this way because it frequently upsets users to have old topics revived when they are no longer relevant. It can also be frustrating to the person who revived the thread when they don’t get a reply, because the original poster is no longer active or no longer finds the topic relevant. Feel free to start a new thread on this topic.


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