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Example: Diltiazem (Cardizem) 60 mg PO b.i.d. On hand are 30 mg tablets.

Ratio and Proportion Method:

Prescribed: Amoxicillin 100 mg PO q.i.d.

Available: Amoxil (amoxicillin) 80 mL

250x = 500

x = 2mL

Answer: Amoxicillin 100 mg = 2 mL

Prescribed: Aspirin/ASA gr x, q4h, PRN for headache

Available: Aspirin 325 mg/tablet

a. Convert to one system and unit of measure. Change grains to milligrams

b.

Be sure to label all the terms in the formula with the appropriate unit. This
will help to prevent errors. For example, use mg following a value in the formula
if the value is in milligrams.

Parenteral Medications

Parenteral medication is a medication that is administered to a patient by an
injection or by an intravenous flow. The dose for an injection is calculated using
the formula method or the ratio-proportion method that is described previously
in this chapter.

325 mg : 1 tab : : 600 (650) mg : x

Means

Extremes

325 x = 600 (650 mg)

x = 1.8 tablets or 2 tablets

(round up or use 650 instead of 600)

250 : 5 mL : : 100 mg: x mL

Means

Extremes

60

30

2

mg

mg

×=tablet tablets

CHAPTER 7 Dose Calculations^119