Introduction

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.

Tech Note

When using this formula to calculate a dose of medication for an oral liquid order, the dose to be given will be in a liquid form such as milliliters (mL) or cubic centimeters (cc). Also remember that if the dose desired and the dose available or on hand are not in the same measurement system, a conversion to the system of the dose available should be made. (See Chapter 4 to review the measurement systems and Chapter 5 for help with conversions between systems.)

Example 8-1

A physician orders amoxicillin 500 mg oral suspension. The medication available is amoxicillin 250 mg/5 mL. How much medication would be administered to the patient for a single dose?

Remember the formula for using ratio and proportion appears as follows:

< ?xml:namespace prefix = "mml" />DA:DF::DO:DG

If you prefer using fractional units for the relationships between the ratios in the proportions, the formula to be used is as follows:

DADF=DODG

Now that you have reviewed the formula, you can calculate the problem given because all of the given measurements are in the metric system.

The DA is 250 mg in the DF of 5 mL. The DO is 500 mg. The proportion is set up as follows:

250mg(DA):5mL(DF)::500mg(DO):x(DG)

OR

250mg(DA)5 ml(DF)=500mg(DO)x(DG)

Either multiply the means versus the extremes or cross-multiply if using the fractional components.

As previously learned, drop the mg because it appears in both sides of the equation, so the equation now appears as follows:

250x=500×5mL

x=500×5 mL÷250or2500mL÷250

x=10 mLor10mL is the prescribed amount of amoxicillin to beadministered to the patient

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.

Example 8-2

A physician orders Ceftin oral suspension 187 mg. The medication on hand is Ceftin 125 mg/5 mL. Because this prescription will be given at home, the medication order for the label should read the amount to be given in household measurements. Again, review Chapters 4 and 5 as needed.

125 mg(DA):5mL(DF)::187mg(DO):x(DG)

OR

Again, either multiply means and extremes or cross-multiply the fractional equivalents as previously accomplished. The resultant formula follows:

125×x=187×5 mLor125x=935 mL

x=7.48or7.5 mL after rounding

Tech Note

Round all doses to a measurable quantity for the utensil being used.

Now change this to the household measurement as requested by the physician.

5mL:1tsp::7.5 mL:x

5x=7.5

Remember that the “mL” designation may be removed because it appears in both equivalents following cross-multiplication.

xtsp=7.5÷5or112

Therefore the dose to be given in household measures is tsp. Remember that household measurements are in fractional units rather than decimal units.

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:

Example 8-3

A physician orders amoxicillin 500 mg oral suspension. The dosage available is amoxicillin 250 mg/5 mL. How much medication would be administered to the patient for a single dose?

xmL=5mL×500250or2500 mL250 

x=10 mL

Example 8-4

A physician orders Ceftin 187 mg oral suspension. The medication on hand is Ceftin 125 mg/5 mL. Because this prescription will be given at home, the medication order for the label should read the amount to be given in household measurements. With dimensional analysis the problem will have an unknown in teaspoons (x = teaspoons). To obtain the answer in teaspoons, one step of fractional components is added to the calculation, thus eliminating the need for two separate calculations as was necessary with the ratio and proportion or formula methods.

x tsp=187mg1dose×5mL125mg×1tsp5mL

x tsp=187×1125×1

x = 1.49 tsp or 1.5 tsp after rounding to a measurable quantity

The dose to be given is tsp.