Pharmacokinetics....need some help!

[farmacology]

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Heya people,

If you double the dose of a drug, its duration of action in the body is only increased by one half-life.

I think I understand this concept, but in trying to answer the following question but I run into some weird conceptual problems.

The question:


.The half-life of drugA is 8 hours. If a person takes 1000 mg of this drug it falls below its clinically effective range 12 hours after ingestion. In general, if the patient wants this drug to be effective for 36 hours, how much should the patient take?.

I know this seems like a simple problem but, I am not sure if I am doing it right.

My calculations:

halflife= 8 hrs
1000mg drug effective for= 12 hours
how much drug needed, in mass, to be effective for 36 hours?

1000mg-->2000mg--->4000mg--->8000mg
12hrs------>8hrs--------->8hrs------->8hrs

So, if I double the drug dosage from 1000mg to 2000mg, I would only end up increasing the duration of action by one half life, which is 8 hours. If I double the dosage from 2000mg to 4000mg, it would again increase the duration by only 8 hours.

Following the reasoning above, the person would need to take roughly 8000mg of drug in order for it be effective for 36 hours.

This is what I think, but I don't think I am doing it right. I am not sure if I should use an equation since I can't find one that would be adequate for this question.

Thoughts?

 

[farmacology]

bummmp.

 

[cycloketocaine]

bummmp.

Seriously?

 

[farmacology]

What?.....

 

[xiphoid2010]

Heya people,

If you double the dose of a drug, its duration of action in the body is only increased by one half-life.

I think I understand this concept, but in trying to answer the following question but I run into some weird conceptual problems.

The question:


.The half-life of drugA is 8 hours. If a person takes 1000 mg of this drug it falls below its clinically effective range 12 hours after ingestion. In general, if the patient wants this drug to be effective for 36 hours, how much should the patient take?.

I know this seems like a simple problem but, I am not sure if I am doing it right.

My calculations:

halflife= 8 hrs
1000mg drug effective for= 12 hours
how much drug needed, in mass, to be effective for 36 hours?

1000mg-->2000mg--->4000mg--->8000mg
12hrs------>8hrs--------->8hrs------->8hrs

So, if I double the drug dosage from 1000mg to 2000mg, I would only end up increasing the duration of action by one half life, which is 8 hours. If I double the dosage from 2000mg to 4000mg, it would again increase the duration by only 8 hours.

Following the reasoning above, the person would need to take roughly 8000mg of drug in order for it be effective for 36 hours.

This is what I think, but I don't think I am doing it right. I am not sure if I should use an equation since I can't find one that would be adequate for this question.

Thoughts?

why not just double check your reasoning with the simple math?

C = Co e^(-KeT)

you can find Ke using: T1/2 = ln 2/Ke

In this case, T is the additional time (24 hours), you know C = 1000, you are solving for Co.

 

[farmacology]

Thanks xiphoid, I was able to get the right answer using that equation.

I actually used that equation to verify my answer, but I was solving for C not Co...

This helps a lot!

 

[farmacology]

Also, is using the formula above the only way to do it since I just added the numbers in my original post to get to the right answer?

 

[xiphoid2010]

Also, is using the formula above the only way to do it since I just added the numbers in my original post to get to the right answer?

These 2 formulas are essential to one compartment 1st order kinetic questions (which your problem is). So you definitely will need to know them like the back of your hand and be able to use them in your sleep before it's over. There are zero order and 2nd order equations you'll have to learn as well, but that's beyond the scope of the topic.

Once you get into 2 or more compartment models, formulas will get pretty complicated. Luckily you won't be doing many of those in real life.

 

[optom123]

These 2 formulas are essential to one compartment 1st order kinetic questions (which your problem is). So you definitely will need to know them like the back of your hand and be able to use them in your sleep before it's over. There are zero order and 2nd order equations you'll have to learn as well, but that's beyond the scope of the topic.

Once you get into 2 or more compartment models, formulas will get pretty complicated. Luckily you won't be doing many of those in real life.

Thanks man, appreciate the help.

 

[optom123]

Wow certainly looks complicated!