1. Oral Dosage of Drugs
Chapter 10

2. Dosage Calculations Ratio and Proportion Method
Step 1. Convert
Ensure measurements are in same system and units are in same size.
If not, use ratio and proportion to convert.
Step 2. Think
Estimate what is a reasonable dose.

3. Dosage Calculations Ratio and Proportion Method
Step 3. Calculate
Calculate drug dose using a proportion.
Look for a known ratio [usually the dosage on hand]
Set up a proportion
Known ratio is equivalent to ratio for desired drug.

4. Converting to Same System Ratio and Proportion Method
Ordered: V-Cillin 500 mg
Available: V-Cillin 0.5 g per tablet
What do you notice?
Same system
Different size units
Needs to be converted

5. Converting to Same System Ratio and Proportion Method
Math Tip
In most cases, it is more practical to convert to smaller unit.
Usually eliminates decimal or fraction, keeping calculation in whole numbers.

6. Converting to Same System Ratio and Proportion Method
Equivalent: 1g = 1,000 mg 1 g . 5 g = 1, m g X m g X = 1, . 5 X = 5 m g

7. Converting to Same System Ratio and Proportion Method
Let’s review order
Ordered: V-Cillin 500 mg
Available: V-Cillin 0.5 g per tablet
After converting, what is ordered and available are the same amount:
500 mg

8. Converting to Same System Ratio and Proportion Method
Ordered: Phenobarbital gr p.o.q. 12 hr
Available: Phenobarbital 15 mg per tablet
What do you notice?
Different system
Needs to be converted

9. Converting to the Same System Ratio and Proportion Method
Math Tip
Convert apothecary and household measurements to their metric equivalents
Metric system is the predominant system of measurement for drugs

10. Converting to the Same System Ratio and Proportion Method
Equivalent: gr 1 = 60 mg g r 1 g = = 6 m g X m g X - 6 X = 6 m g g r = g r = 3 m g

11. Converting to Same System Ratio and Proportion Method
Let’s review order
Ordered: phenobarbital gr p.o.q. 12 hr
Available: phenobarbital 15 mg per tablet
After converting
30 mg was ordered

12. Converting to Same System Ratio and Proportion Method