Math for Medications Some terms you’ll see: Desired Dose Available Dose Ratio & Proportion Cross Product

Math for Medications The purpose of this class is for the learner to be able to calculate drug dosages of tablets and liquids. You will calculate the drug dosages using the formula or ratio & proportion method.

Math for Medications- the Formula Method for Tablets Desired doseX Vehicle = ATA Available dose ddxv = ATA ad(amount to administer)

Problem The doctor ordered Benadryl 75 mg. The drug label reads Benadryl grams. How many tablets are needed? Read the problem and identify what you’ve been given.

Problem dd = 75 mg ad= grams v = 1 tab Have both dd & ad in the same unit of measure g = 25 mg. ad = 25 mg

Problem Put the numbers into the formula: 75 mgx1 tab =3 tabs 25 mg

Math for Medications- The Ratio & Proportion Method for Tablets We will use the same problem but will cross multiply.

Problem The doctor ordered Benadryl 75 mg. The drug label reads Benadryl grams. How many tablets are needed? Read the problem and identify what you’ve been given.

Problem The known ratio: 25 mg 1 tablet The unknown ratio: 75 mg N tablets

Problem Write the proportion : 25 mg x 75 mg 1 tablet N tablets

Problem Cross Multiply: 25 mg x 75 mg 1 tablet N tablets 25mg X N tab = 1 X 75 mg

Problem Solve for N by dividing both sides of the equation by 25: 25 N=7525 or N = 3

Problem Substitute 3 for N in the original proportion and your answer is : You would administer 3 tablets to give a dosage of 75 mg.

Math for Medications for Liquids Calculate drug dosages using the formula method or ratio & proportion method for liquids.

Problem The doctor has ordered Gentamycin Sulphate 25 mg. The label reads Gentamycin Sulphate 40 mg/mL. How much Gentamycin will you administer?

Problem - Using the Formula Method Identify: dd = 25 mg ad = 40 mg v = 1 mL Put the numbers in the formula: 25x 1 mL= mL 40

Problem Round off to the nearest decimal place: 25x 1 mL= mL or 0.6 mL 40

Problem- Using the Ratio & Proportion method Cross product ( Cross Multiply) : The know ratio:40 mg 1 mL The unknown ratio:25 mg N mL

Problem Proportion:40 mg =25mg 1 mLN mL Cross Multiply: 40 x N = 1 x 25

Problem Solve for N: 40N= N =0.625 Round off: N = 0.6 mL

Rules to Remember Substitute for N in the original proportion: Proportion:40 mg =25mg 1 mL0.6 mL

Rules to Remember Example: 0.66mg = 0.7 mg 1.Put 0 to the left of the decimal if there is no whole number. 2.Adults: round of drug doses to the nearest 1/10 or Pediatrics: Round off drug doses to the nearest 1/100 or 0.01

Rules to Remember Do not round off until your final answer: For example:100 mg x 15 mL 80 mg 10=1.25 x 15 mL = mL 8 Not 1.3 x 15 mL which would =19.5mL

References SIAST. (2006). PHAR 264 – Administration of medications [Coursepack]. Regina : SIAST Wascana Campus.