Calculate Doses of Oral and Parenteral Liquid Medications

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Chapter 8

Calculate Doses of Oral and Parenteral Liquid Medications

Key Words


Interpret each medical order that follows. Show all of your mathematical calculations. Round answers to nearest tenth.

image 1 A physician orders Ceclor 250 mg po tid for a child with otitis media.

image 2 A physician orders Atarax Syrup 15 mg po qid.

image 3 A physician orders Tylenol gr v q4h prn fever and aches for a child who has a high fever. The drug available is Tylenol elixir 160 mg/tsp.

image 4 A physician orders Amoxil 62.5 mg po tid for an infant. The drug available is 125 mg/5 mL.

image 5 Using the medication order in question No. 4, what amount of medication should be administered in the household measurement? ____________________

image 6 A physician orders Zofran 2 mg IM to be given 30 minutes before the treatment for a chemotherapy patient. The drug dosage available is Zofran 2 mg/mL.

image 7 A physician orders Ticar 500 mg IM bid. The reconstituted strength of the medication is 1 gm/2.6 mL.

image 8 The physician orders vitamin B12 1000 mcg IM qwk. The available drug dosage is 1 mg/mL.

image 9 A physician orders meperidine 75 mg IM stat for a patient who has had cardiac surgery. The drug dosage available is meperidine 50 mg/mL.

image 10 A physician orders 50 mg of meperidine and 25 mg of promethazine IM q4-6h prn for a postsurgical patient with pain and nausea. The drug dosage available is meperidine 75 mg/mL and promethazine 25 mg/mL.

image 11 Ordered medication: amoxicillin 375 mg po tid


image 12 Ordered medication: streptomycin 750 mg IM


image 13 Ordered medication: codeine phosphate gr i subcutaneous q4h prn


image 14 Ordered medication: Mycostatin 500,000 units po tid


image 15 Ordered medication: erythromycin 0.3 gm po tid


image 16 Ordered medication: phenobarbital 30 mg po hs*


image 17 Ordered medication: Prozac 10 mg po qam


image 18 Ordered medication: Benadryl 25 mg po prn


image 19 Ordered medication: morphine sulfate gr 1/10 IV q4h


image 20 Ordered medication: Colace syrup 80 mg po at bedtime


*These abbreviations are found on the TJC Do Not Use List and ISMP’s List of Error-Prone Abbreviations, Symbols, and Dose Designations due to medication safety issues. They should not be used. You are being tested on them here because these abbreviations may still appear in the pharmacy setting.


For some patients, solid medications such as tablets and capsules are difficult to swallow, and the physician will order oral liquid preparations. These preparations are usually stated in weight or strength/volume such as milligrams/milliliter. Because liquid medications may be dispensed in either drams from the apothecary system or teaspoons in the household measurement system, conversions among all three measurement systems are often required for dispensing or administering the prescription as written. As with solid medications, these conversions may be accomplished by either ratio and proportion or dimensional analysis. If ratio and proportion is used, then the formula method may be used for preparation of the medication for dispensing. If dimensional analysis is used, the conversion among systems may be accomplished by use of additional ratios for conversion in the equation. Thus the entire conversion and calculation may be completed with one elongated proportional step. (Examples are shown later in this chapter.) Some medications for oral administration are prepared in a powder form for reconstitution to a liquid form before dispensing—the subject of Chapter 9.

Injectable medications may be given when a person is unable to swallow solid medications or when a quicker effect is necessary. Parenteral medications are given by injection into body tissue and may also be given intravenously or directly into the bloodstream. Because solid medications cannot be injected under the skin except as special intradermal forms such as pellets, most injectable medications must be in liquid form. As with oral liquid medications, some parenteral medications come in powders for injection, requiring reconstitution before administration. These medications usually will not be stable in liquid form for an extended period, so they must be handled for stability at the time of administration (reconstitution of parenteral medications is discussed in Chapter 9).

Calculation Of Oral Liquid Medications

Oral medications are available in solid form as discussed in the previous chapter and in liquid form as discussed in this chapter. When oral medications are given in a liquid form, the absorption is usually faster because solids have to dissolve before absorption. The absorption of a medicine is controlled, in part, by the dosage form administered. Most oral medications are absorbed in the small intestine, although some are absorbed beginning in the mouth or stomach. These medications may be administered with oral syringes that are available in 1-mL, 5-mL, and 10-mL sizes (Figure 8-1), a medication cup (Figure 8-2), a dropper that is available in different measurements and is usually tailored by the manufacturer for a specific drug (Figure 8-3), dosage spoons (Figure 8-4), or by using household devices such as teaspoons and tablespoons. The newest form of administration of oral liquid medications is through a nipple or with a pacifier that will hold a measured amount of medication (Figure 8-5). The choice of administration implement for oral liquid medications depends on the volume of medication to be administered and the availability of supplies. Medication cups are found in institutional settings, whereas droppers and oral syringes may be used either in institutions or at home. Teaspoons and tablespoons are the measurements occasionally used for larger doses in the home setting or when the calibrated utensils are not available. The use of kitchen utensils is usually discouraged, because oral syringes and calibrated cups are more accurate.

Liquid medications may be found in tinctures, elixirs, suspensions, and syrups, some of which may be irritating to the gastric system and must be diluted before administration. Some medications such as cough syrups should not be diluted even after the administration; therefore the information about the medication and its use and proper administration is necessary before administering a dose or providing patient education.

Liquid drug preparations are labeled with the strength of the drug (such as milligrams) in the total volume (such as milliliters) of the liquid. Remember that liquid medications may be expressed in metric (milliliters, cubic centimeters, and liters), apothecary (drams and minims), and household (teaspoons, tablespoons, pints, and the like) measurement systems. Doses of medication are also calculated based on strength/volume. This fact is then used to prepare liquid medications for administration. For example, a physician may order amoxicillin 500 mg to be given to a child who requires a liquid dosage form. The medication on hand is amoxicillin 250 mg (weight or strength) per 5 mL (volume). In this case the known amounts will be placed into the formula, ratio and proportion, or dimensional analysis equations for preparing the correct amount of medication. If you need to review the formulas for any of these ways of ascertaining the correct dose to be given or if you need to review the safety measures necessary for medication preparation and administration, return to Chapter 7.

As with the previous chapter, you will calculate doses for competency. You must remember that dose is the amount of medication to be administered at a specific given time, whereas dosage (to be discussed in Chapter 14) is the total amount of medication that will be administered over a particular length of time. Again, as a reminder, the label on the medication must be in the same system of measurement as that found on the prescription or medication order. If these do not agree, conversion should be accomplished to the system found on the label. Also, be sure the medication names are exactly the same, because many drug names look and sound alike.

Calculating Oral Liquid Medications Using Ratio and Proportion

As you learned in Chapter 7, when using the ratio and proportion method of drug calculation, the dose available (DA) and the dosage form (DF) must be one ratio, and the dose ordered (DO) and dose to be given (DG) must be the other ratio in the proportion. With liquids, DF and DG will be in volume, whereas DA and DO will be in strength or weight of drug.

Try another medication order as an example. Because most oral liquid medications must be calculated to the metric system, the examples are in the metric system.

Calculating Oral Liquid Medications Using Dimensional Analysis

Remember that dimensional analysis is really just an elongated form of ratio and proportion using fractional components. To review dimensional analysis as a means of calculating doses, please review Chapter 5. The problem in Example 8-1 would appear as follows for calculation with dimensional analysis: