Half Life Question?
by Mariya
(Las Vegas, NV)
How do you figure out this question from the practice exam? If a Tadalafil 10mg/tab has a typical half life of 17.5 hours, how long will it take until the blood concentration of this dosage falls to 25% of its initial strength ?
Reply:(Keith)
The term Halflife refers to the time it takes for the concentration of a drug in your system to drop by 50% (Half).
There's a page on HalfLife here
Now, let's say at 5pm a man takes a Tadalafil 10mg/tab which has a typical half life of 17.5 hours. He will have 10mg in his system. Now, If his blood was drawn and tested after 17.5 hrs it would show that "Half" of the drug has been metabolized / eliminated.
So, At that time he will have approximately 5mg of the drug left in his system(Half). Consequently, in another 17.5 hours, half of that remaining 5mg will be eliminated. At that time, he will have about 2.5mg of the drug in his system. What this tells us is that after 35 hours (in this case), or 2 half lives, the concentration of the drug will be at 25%.
100%  At time of dosing.
50%  After first half life periods.
25%  After two half life periods.
In first order / linear kinetics, your body gets rid of half of however much of the drug is in your system every half life interval. That is why it's called a halflife.
The rate at which you body breaks down these types of drugs is relative to the concentration of the drug in your system, not an absolute rate of elimination.
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