IV to PO conversion

[pharmDNC]

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Having this debate with a people...and can't seem to find the answer yet.

Patient is given Drug X 500mg IV. Now patient is given Drug X again, but this time it is given PO. Drug X has a bioavailability of 70% via oral route. So what is the new oral dose of Drug X?

Any suggestions?

 

[SpirivaSunrise]

Having this debate with a people...and can't seem to find the answer yet.

Patient is given Drug X 500mg IV. Now patient is given Drug X again, but this time it is given PO. Drug X has a bioavailability of 70% via oral route. So what is the new oral dose of Drug X?

Any suggestions?

Well, think about it for a minute. Assuming this is a simple kinetics question and you aren't getting too technical with complicated disease states/patient variables...you can assume the absolute bioavailability when given IV is 1 correct? It's all reaching systemic circulation. Now...when you're giving it extravascularly, F is going to be < 1 (due to various physiological factors). In this case, 0.7. So if less of the drug is reaching systemic circulation, would it make sense that you'd have to give more or less of the drug? And how much more or less?

 

[acetyl]

Having this debate with a people...and can't seem to find the answer yet.

Patient is given Drug X 500mg IV. Now patient is given Drug X again, but this time it is given PO. Drug X has a bioavailability of 70% via oral route. So what is the new oral dose of Drug X?

Any suggestions?

500mg/70%

 

[DrugDealer]

[PO dose x PO bioavailability] = [IV dose x IV bioavailability]

X x 0.70 = 500 mg x 1

X = 500 mg/0.70

 

[WVUPharm2007]

But watcha gonna do if it don't come in 715s......?!?!?!?!

 

[SpirivaSunrise]

Well, I hope you get the concept too pharmDNC! And the math...because I'm sure a lot of people would say 650 mg.

 

[DrugDealer]

But watcha gonna do if it don't come in 715s......?!?!?!?!

Pray that it comes in a suspension so I can get every last milligram of that 715! 😎

 

[gotdrugs]

There are other factors than the listed bioavailability of a drug.

Example:

How many of you have IV to PO conversions of Zithromax at your hospital and if you do, Do you increase the Dose?