Dosage Calculations

1 Dosage CalculationsAfter completing this chapter, youshould be able to: Solve one-step pharmaceuticaldosage Calculations . Set up a series of ratios andproportions to solve a singledosage calculation . Determine what informationyou will need to solve for, inaddition to any giveninformation, to properlycalculate dosages. Convert pediatric weights frompounds to kilograms. Accurately determine dosagesbased on mg/kg/day. Define common sig (signa)codes used on dosing of medications is important to ensure patientsafety. calculating dosages, Dosage regimens, andcompounding formulas involves the use of simple mathprinciples. You can solve many of these problems by settingup ratios and proportions using the information given in thequestion and keeping like units 8/31/07 2:39 PM Page 2 Chapter Five Dosage Calculations3 Sig RefresherThe sigportion of the prescription order, meaning signa, is where the instruc-tions for the patient are written.

2 Pharmacy technicians enter the informationfrom the prescription order into the computer. The sig is an important valueto remember in order to properly determine pediatric dosages. The followingare some of the more common sigs you will find on prescriptions:qdevery dayqodevery other dayddaily bidtwice a daytidthree times a dayqidfour times a dayq4hevery 4 hrq6hevery 6 hrq8hevery 8 hrq4 6hevery 4 6 hrprnas neededDepending on the workplace, you may also see sigs such as the following:q3devery three daysqmwfevery Monday, Wednesday, and Fridayqwevery weekDosage CalculationsDosage Calculations include calculating the number of doses, dispensingquantities, and ingredient quantities; these Calculations are performed in thepharmacy on a daily basis. The pharmacy technician must have a full work-ing knowledge of how to perform these perform Dosage Calculations , you will utilize the information andprinciples introduced in the previous chapters of this book.

3 You can solvethese Calculations by setting up ratios and proportions, keeping like unitsconsistent, and THE NUMBER OF DOSESTo calculate the number of doses, you should first determine which informa-tion presented is actually applicable to the question. Too often mistakes aremade on Dosage Calculations because we overcomplicate many 1-tsp doses are in a 4 oz bottle of Prozac LiquidSolution 20 mg/5 mL?Rx Prozac Solutiontsp. po qdDisp. # 4 8/31/07 2:39 PM Page 34 Chapter Five Dosage CalculationsLet s look at the information that has been provided: tsp po the dose qd the frequency 4 oz the quantity dispensed Prozac Solution 20 mg/5 mL the drug name and strength 120 mL the quantity of the stock bottleThe question is simply asking how many doses make up the total amountbeing dispensed. The strength of the drug, frequency of Dosage and quantityof the stock bottle have no relevance in performing this now we know that we are working with 1 tsp.

4 Doses and a total quan-tity of 4 oz, but to solve this calculation using a ratio/proportion we have tohave similar units of measure in this case know that 1 tsp = 5 mL and we should also know that 4 oz. = 120 mL,but if you didn t that could also be solved by using a and solve the equation for .Now that you have both quantities converted to units in mL, we can setup our ratio/proportion and *x=5x and 1*120=1201 dose5 mL=x doses120 mL(1)x=12030*4=120 and 1*x=(1) xx1 oz30 mL=4 ozx 5-1 Drug label for Prozac.(Courtesy of Eli Lilly and Company.) 8/31/07 2:39 PM Page 4 Chapter Five Dosage Calculations5 Now set up your equation and solve for solve for x, divide both sides by , there are 24 doses (of 5 mL) in a 4 oz (120 mL) THE NUMBER OF DOSESEXAMPLE many doses are provided in the prescription below?Rx Ibuprofen 400 mg-po q 6 hr prn painDisp. # 120 Let s look at the information that has been provided: - po the dose q 6 hr prn pain the frequency 120 the quantity dispensed Ibuprofen 400 mg the drug name and strengthWorkplace WisdomAlways use the higher Dosage amount when perform-ing Dosage Calculations on prescriptions that have a range for the dose, as inExample This will provide the most conservative solution and ensurethe most accurate potential for days the information provided, set up the ratio/proportion and set up your equation and solve for.

5 To solve for , divide both sides by , there is a minimum of 60 doses prescribed. x=60 120,2=60 2x2=1202x2x=120x2*x=2x and 1*120=1201 dose2 tabs=x doses120 x=24 120,5=24 2x2=12025x= 8/31/07 2:39 PM Page 56 Chapter Five Dosage CalculationsPRACTICE PROBLEMS many dosages are provided in the prescription listed below?_____Rx Cardura 2 mgpo qdDisp. # many droppersful (a mL Dosage ) are in a 50 mL bottle ofEryPed Drops? many dosages are provided in the prescription listed below?_____Rx Vicodin 5/500po q8hrDisp. # many 1 tsp doses are in each bottle of Zithromax 200 mg/5 mL aslisted below, when mixed? many dosages are provided in the prescription listed below?_____Rx Regular Insulin U-100 (100 units/mL)Inject 20 units qAMDisp. # 1 vial (10mL)Figure 5-2 Drug label for EryPed Drops.(Reproduced with permission of Abbott Laboratories.)Figure 5-3 Drug label for Zithromax.(Registered Trademark of Pfizer Inc. Reproduced with permission.)

6 8/31/07 2:39 PM Page 6 Chapter Five Dosage Calculations7 calculating THE QUANTITY TO DISPENSEEXAMPLE many Biaxin 250 mg tablets should be dispensed?Rx Biaxin 250 mgpo BID 5-4 Drug label for Biaxin.(Reproduced with permission of Abbott Laboratories.)Again, to solve this Dosage calculation it is important to first determinewhich information is necessary; it is also critical to know the common SIGcodes to perform Dosage s look at what information that has been provided: po the dose BID the frequency x10d the duration Biaxin 250 mg the drug name and strength 100 tablets the quantity of the stock bottleTo calculate the appropriate quantity to dispense, use the following formula:dose frequency duration quantity to dispenseUsing the information provided in the prescription, you can set up thecalculation as such:1 2 10 xdose frequency duration quantity to dispense1 2 10 2020 xSo, 20 tablets of Biaxin 250 mg should be 8/31/07 2:39 PM Page 78 Chapter Five Dosage CalculationsCALCULATING THE QUANTITY TO DISPENSEEXAMPLE much Promethazine w/Codeine syrup should be dispensed?

7 Rx Promethazine w/Codeine Syrup po QID 4dLet s look at what information has been provided: tsp. po the dose QID the frequency x4d the duration Promethazine w/Codeine Syrup the drug name and strengthTo calculate the appropriate quantity to dispense, use the following formula:dose frequency duration quantity to dispenseUsing the information provided in the prescription, you can set up thecalculation as such:1tsp 4 4 xdose frequency duration quantity to dispense1 4 4 1616 xSo, 16 teaspoonsful, or 80 mL, of promethazine w/codeine should PROBLEMS quantity should be dispensed for the prescription listed below?_____Rx Zoloft 50 mgpo qdDisp. 1 mo. quantity should be dispensed, using the stock medication below,to provide 20 mg of diazepam prior to the procedure and 10 mgfollowing? 5-5 Drug label forDiazepam.(Used with permission ofRoxane Laboratories, Inc.) 8/31/07 2:39 PM Page 8 Chapter Five Dosage quantity should be dispensed for a 30-day supply of the followingprescription?

8 _____Rx Soma 350 mgpo BID TID prn muscle spasmsCALCULATING THE QUANTITY OF INGREDIENTEXAMPLE much codeine is in each dose of Codeine Phosphate OralSolution 15 mg/mL?Rx Codeine Oral Soln. 15 mg/mLtbsp po prn painDisp. # 4 5-7 Drug label forCodeine Phosphate OralSolution.(Used with permission ofRoxane Laboratories, Inc.) quantity should be dispensed for a three-month supply of thefollowing prescription? _____Rx furosemide 20 mgpo quantity should be dispensed if 50 mg of amitriptyline has beenprescribed daily for 3 weeks, if the medication listed below is all that isavailable? 5-6 Drug label for Amitriptyline HCL.(Courtesy of Geneva Pharmaceuticals.) 8/31/07 2:39 PM Page 910 Chapter Five Dosage CalculationsLet s first look at all of the information provided to determine which willbe used in solving the problem: tbsp po the dose prn the frequency 4 oz the quantity to dispense Codeine Phosphate Oral Solution 15 mg/mL the drug name and strength 500 mL the quantity of the stock bottleTo solve this problem, we need to set up a ratio/proportion using the doseand strength, but again remember that units of measure must be the dose (1 tbsp) is equivalent to 15 mL which you should we can set up the and then set up the equation to solve for divide both sides by 5 to solve for , the answer is 45 mg of codeine in each 1 tbsp PROBLEMS the following Dosage with codeine syrup contains mg promethazine and10 mg codeine per 5 mL.

9 How many milligrams of promethazine are in2 tsp? hydroxycobalamine 10,000 mcg/cc 30 cc Sig: 1 cc IM hsHow many milligrams are in each dose? acetaminophen with codeine#3 acetaminophen 300 mg/codeine 30 mg30 tabletsHow many grains of codeine are in each tablet? _____CALCULATING THE QUANTITY OF INGREDIENTEXAMPLE many mL of stock Dopamine must be added to the IV solution?Rx Dopamine 400 mg added to 500 mL of NSStock: Dopamine HCl Injection 80 mg/mL x=45 5x5=2255 5x=225 15*15=225 and 5*x=5x15 mg5 mL=15 mg1 8/31/07 2:39 PM Page 10 Let s first look at all of the information provided to determine which willbe used in solving the problem: 400 mg the dose 500 mL the quantity to dispense Dopamine HCl Injection 80 mg/mL the drug name and strengthTo solve this problem, we must determine how many milliliters of the stockDopamine will need to be added to the normal saline IV solution must set up a and then set up the equation to solve for divide both sides by 80 to solve for , the answer is 5 mL of the stock Dopamine HCl Injection must beadded to the IV PROBLEMS s contain 6 tablets of azithromycin 250 mg, which is taken overthe course of five days.

10 How many total mg of active ingredient arecontained in a Z-Pak ? many mcg of Fentanyl would be contained in mL?_____ x=5 40080=80x80 400=80x 1*400=400 and 80*x=80x80 mg1 mL=400 mgx mLChapter Five Dosage Calculations11 Figure 5-8 Drug label forFentanyl.(Courtesy of ESI Lederle, a BusinessUnit of Wyeth Pharmaceuticals,Philadelphia, PA.) 8/31/07 2:39 PM Page many milligrams of acetaminophen are contained in 2 Lortab ( mg hydrocodone/500 mg acetaminophen) tablets ? many milligrams of dexamethasone are contained in the stockbottle (500 mL) below? _____12 Chapter Five Dosage many mg of hydrocortisone are found in 1 tbsp of Cortef 10 mg/5 mL? _____Figure 5-10 Drug label forCortef.(Courtesy of PharmaciaCorporation.)Figure 5-9 Drug label forDexamethasone Oral Solution.(Used with permission of RoxaneLaboratories, Inc.) calculating THE CORRECT DAYS SUPPLYEXAMPLE many days should the following prescription last?Rx Pamelor 75 mgpo BIDDisp.