# 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

 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 - - - - - - - 150mLX = 4500mgSo, 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% 100mLor you could have used: 300mg---------- = 3% 10mLor even: 30mg---------- = 3% 1mLYou'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 = 4.5ml1g = 1ml500mg = .5g4.5ml / .5g = 9

 Confused by: Anonymous I have the same question and an answer key but not an explanation of how to get to it. The reason why I am writing is the answer key is showing the answer to be 10 tablets not 9. "How many 500 metronidazole tablets will be needed to compound the following prescription for a patient: metronidazole 3%, suspending agent 30%, simple syrup 40% qsad H2O to 150ml?" And please, yes, break everything down. Step by step because I am not understanding this.