Lecture Math for the pharmacy technician: Concepts and calculations: Chapter 7 – Lynn M. Egler, Kathryn A. Booth

Chapter 7, Oral medications and parenteral dosages, discusses tablets and capsules in depth and gives information about breaking or crushing them. Liquid oral medications are also discussed. Chapter 7 applies techniques learned in Chapter 6 to calculations of parenteral dosages, emphasizing injectable medications. The chapter concludes with a look at other medication routes such as eye and ear drops, inhalants, rectal and vaginal medications, transdermal systems, and topical medications.

Oral Medications and Parenteral Dosages

 Calculate the amount of a parenteral 

medication to administer.

When you have successfully completed Chapter 7, you will have  mastered skills to be able to:

Introduction

 You will learn to apply the simple calculation 

methods learned in Chapter 6 to oral dosages,  including solids and liquids.

 Follow the method of calculation that you have 

chosen by following the color coded examples.

Remember that excellence is a must with 

dosage calculations.

Calculating Dosages for Tablets  and Capsules

 Always question and/or verify when your 

calculation indicates to give a portion of a  tablet when the tablet is not scored.

 Question and recheck any calculation that 

indicates that you should dispense more  than three tablets or capsules.

Calculating Dosages for Tablets  and Capsules (con’t)

Calculating Dosages for Tablets  and Capsules (con’t)

Fraction Proportion Method

Calculating the Amount to Dispense by Fraction

Proportion:

1 Set up the proportion as follows:

dosage unit amount to dispense

2 Cancel units.

3 Cross multiply, then solve for the unknown value.

Ratio Proportion Method

Calculating the Amount to Dispense by Ratio Proportion:

1 The proportion will be set up as follows:

dosage unit : dose on hand :: amount to dispense : desired

With dimensional analysis you will not need to calculate the

desired dose and amount to dispense separately

You will place your unknown (amount to dispense) on one side

of the equation then multiply a series of factors on the right side of the equation.

Canceling units will help you determine the equation has been

set up correctly.

Dimensional Analysis Method  (con’t)

Calculating the Amount to Dispense by Dimensional

Analysis (con’t):

1 Determine the units of measure for the answer and place it

as the unknown on one side of the equation.

2 On the right side of the equation, write a conversion factor

with the units of measurement for the desired dose on top

and the unit of measurement for the dose on hand.

3 Multiply the conversion factor by a second factor: the

dosage unit over the dose on hand.

4 Multiply by a third factor: dose ordered over the number

one.

5 Cancel units on the right side of the equation The

remaining unit of measure on the right side of the equation should match the unknown unit of measure on the left side

of the equation.

6 Solve the equation.

Formula Method

Calculating Amount to Dispense Using the Formula

Method:

1 Determine the desired dose Determine the dose on hand

(H) and dosage unit (Q)

2 Fill the formula

•D for the desired dose

•H for the dose on hand

•Q for the dosage unit in the formula

•A for the unknown or the amount to dispense

A

xQ

H D   

Formula Method (con’t)

Calculating Amount to Dispense Using the

Formula Method (con’t):

3 Cancel the units.

4 Solve for the unknown.

 If medication cannot be crushed, see if an 

alternative form of the medication exists  and ask the physician to order it that way.

Some Common Food and 

Drug Interactions

effectiveness of  drug

CNS Central Nervous 

System (CNS) 

Depressants

Black cohosh, ginseng,  kava kava, St. John’s  wort, valerian, ETOH

Intensifies  sedative effects 

of CNS  depressant

Decreases  antimicrobial  activity

absorption

Medication: Mixed with: Causes:

excretion Monoamine 

oxidase inhibitors

Foods containing  tyramine such as  hard cheeses,  chocolate, red  wine, and beef or  chicken liver

Headache,  nosebleed,  chest pain,  severe 

hypertension

Some Common Food and Drug 

Interactions (con’t)

Some Common Food and Drug 

Interactions (con’t)

Tetracyclines Dairy products Reduces 

effectiveness 

of drug

Sustained­Release Medications

 Allows the drug to be released slowly into 

the bloodstream over a period of several  hours.

Span Sequel  Tembid 

SA Spansule 

SR Extentab

smallest volume.

Calculating the Volume of Liquid Oral  Medication     

Q A

Calculating the Volume of Liquid  Oral Medication  (con’t)   

 Find the amount to Dispense.

 Ordered: Ventolin 5 mg PO now

 On hand:  Ventolin syrup 2 mg per 5 mL

Use the calculation method of choice.

Fraction Proportion Method

Calculating the Amount to Dispense by Fraction

Proportion:

1 Set up the proportion as follows:

dosage unit amount to Dispense

2 Cancel units.

3 Cross multiply, then solve for the unknown value.

Ratio Proportion Method

Calculating the Amount to Dispense by Ratio

Proportion:

1 The proportion will be set up as follows:

dosage unit : dose on hand :: amount to Dispense : desired

Dimensional Analysis Method

Calculating the Amount to Dispense by Dimensional

Analysis:

With dimensional analysis you will not need to calculate the

desired dose and amount to dispense separately

You will place your unknown (amount to dispense) on one side

of the equation then multiply a series of factors on the right side of the equation.

Canceling units will help you determine the equation has been

set up correctly.

Dimensional Analysis Method

Calculating the Amount to Dispense by Dimensional

Analysis (con’t):

1 Determine the units of measure for the answer and place it

as the unknown on one side of the equation.

2 On the right side of the equation, write a conversion factor

with the units of measurement for the desired dose on top and the unit of measurement for the dose on hand.

3 Multiply the conversion factor by a second factor: the

dosage unit over the dose on hand.

5 Cancel units on the right side of the equation The remaining

unit of measure on the right side of the equation should

match the unknown unit of measure on the left side of the equation.

6 Solve the equation.

Formula Method

Calculating Amount to Dispense Using the

Formula Method:

1 Determine the desired dose Determine the dose on hand (H)

and dosage unit (Q)

2 Fill the formula

•D for the desired dose

•H for the dose on hand

A

xQ

H D   

Formula Method (con’t)

Calculating Amount to Dispense Using the

Formula Method (con’t):

3 Cancel the units.

4 Solve for the unknown.

 Household teaspoons and tablespoons do 

not measure liquid accurately.

Parenteral Dosages

 Parenteral route of administration; medications that 

are delivered outside of the digestive tract; most often  refers to injections. The most common injection types  used to deliver medications include:

mL, use a 0.5­mL tuberculin syringe.

 inhalants

 ophthalmic and otic drops

 topical and  transdermal 

 rectal and vaginal medications.

Estimated Days Supply  (cont.)

 To determine estimated days supply you will 

multiply amount of medication to dispense by  days needed over the number of dosage units  per day. 

Amount to dispense     ? days  = estimated days supply

dosage unitsper day