Pharmacy Compounding Math Question

by Magda
(Frisco, tx)

The Jones' family dog has a parasitic infection. Their Veterinarian has called in an order for 150mL Metronidazole 3% oral susp. How many 500mg metronidazole tablets will be needed to compound the prescription if the recipe is:

Rx:
Metronidazole 3%
Suspending agent 30%
Simple syrup 40%
qs ad H2O to 150 mL

A. 9
B. 10
C. 7
D. 18

Comments for Pharmacy Compounding Math Question

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pharmacy calculations
by: Jerry

The only thing to determine is how many mg of the drug you need. Then divide that number by the contents of the tabs (500mg in this case)

First, use a proportion equation:

3000mg - - - - - - - X mg
----------- = 3% = ---------
100mL - - - - - - - 150mL

X = 4500mg

So, now divide...

4500mg / 500mg (tabs) = How many tabs you need.

OK, Your turn

3000?
by: Anonymous

How did you determine how many mg of the drug you need? e.g. 3000mg

The 3000 is from the 3%
by: Anonymous

The 3000 comes from the fact that

3000mg
---------- = 3%
100mL

or you could have used:


300mg
---------- = 3%
10mL

or even:


30mg
---------- = 3%
1mL

You're using that fraction to cross multiply - since it's a known variable, to find the unknown variable. e.g. - If you know how much is in 100mL, then you can determine what's in 150mL.

Pharmacy Compound Question
by: Brad Wojcik, Pharm D

I think it would be helpful to explain a little about percent strength so the students will know how to calculate the number of mg/ml of a 3% solution. A 3% w/v solution means 3% g/mL. w=g and v=mL. To work with the % sign, just remember that to add the % sign, multiply by 100% and to remove the %, divide by 100%. So, 3% g/mL is the same as 3 g/100 mL.

Different/Quicker
by: Anonymous

You are told in the recipe that the drug is 3% of the content.
You are ordered for 150ml of it.
0.03 (3%) x 150 = 4.5ml
1g = 1ml
500mg = .5g
4.5ml / .5g = 9

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