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A drug achieves a plasma level of 16 mg/L shortly after the administration of the first oral dose. If the half-life and the dosing interval are both 6 hours, what is the approximate plasma level shortly before the administration of the 5th dose?

A. 15 mg/L
B. 24 mg/L
C. 28 mg/L
D. 30mglL
E. 31 mg/L

Please explain your answer.
 

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answer is 15!
because they are asking wats the conc of drug jus before the administration of 5th dose! the conc of drug aftr admin of 4th dose is 30mg.. after half life.. its 15mg.. n thats the conc right before the admn of 5th dose!
 

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Its 15..because steady state concentration is reached after 4th half life which is 95% of the drug..so 95% of 16mg would be 15 mg.
So its concentration before 5th dose is 15mg..
 

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Discussion Starter #9
answer is 15!
because they are asking wats the conc of drug jus before the administration of 5th dose! the conc of drug aftr admin of 4th dose is 30mg.. after half life.. its 15mg.. n thats the conc right before the admn of 5th dose!
Why the conc of drug after 4th dose is 30mg?
 

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i also thnk the answer is 31

with each half life the drug conc decreases by 50% and additional 16 mg gets in..i did the math lke that and it came to 31 mg

is this the right answer?:eek:
 

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No actually 31 isnt the right answer, first i also thot that way then i came to know the correct answer which is 15. . the logic behind is every new dose will be added to the previous remaining dose and then the sum of the two will get halved during each half life and this is true for all doses .. The answer we will get in the end is 15
 

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The key word in this question is before the 5th dose. Immediately "after" the 5th dose, the plasma level should be approximately 30 mg/L, but just before it would be close to half of that level.
 
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