Transcript Chapter 10

Oral Dosage of Drugs
Chapter 10
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.
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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.
Dosage on hand
Dosage desired

Amount on hand XAmount Desired
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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
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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.
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Converting to Same System
Ratio and Proportion Method
• Equivalent: 1g = 1,000 mg
1g
1,0 00 m g
=
0. 5 g
X mg
X = 1,0 0 0  0. 5
X = 50 0 m g
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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
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Converting to Same System
Ratio and Proportion Method
1
2
• Ordered: Phenobarbital gr
p.o.q. 12 hr
Available: Phenobarbital 15 mg per tablet
• What do you notice?
– Different system
– Needs to be converted
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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
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Converting to the Same System
Ratio and Proportion Method
• Equivalent: gr 1 = 60 mg
gr 1
60 mg
X - 60
=

1
2
g
X mg
=
1
2
X = 60 m g
gr
1
2
= gr
1
2
= 30 mg
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Converting to Same System
Ratio and Proportion Method
• Let’s review order
1
– Ordered: phenobarbital gr 2 p.o.q. 12 hr
– Available: phenobarbital 15 mg per tablet
• After converting
– 30 mg was ordered
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Converting to Same System
Ratio and Proportion Method
15 mg 30 mg

1 tab
X
15X  30
15X 30

15
15
X  2 tab
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