Dosage Calculations The elements of the prescription can be curtailed down to several elements Dose is the amount of drug taken at one time Q is a variable that represents the quantity or units that consists of a dose of a medication. i.e. 2 tablet give a 650 mg dose if each dose is 325 mg. F is the frequency of administration per day i.e. BID N is the number of days, or the “day supply” T is the total quantity to dispense Q*F*N=T

Example One Renagel ® Sig: iii TID AC for 30 days with 11 refills. Q= 3, F=TID, N= 30 days therefore T=3*3*30= 270 tablets Renagel® is the brand name of sevelamer chloride which a drug given to patient with end stage renal disease to lower their phosphate levels in the blood.

Example Two i.e. Timoptic® sig: instill 1 gtt OU BID. Dispense 5 ml ophthalmic container. What is the day supply? T= 5ml* 20 gtt/1 ml= 100 gtt Remember Q is the quantity per dose which should take into account the route of administer OU is both eyes so Q=2 gtt Q= 2, F= BID, T=100 what is N? Answer: 2*2*N=100 or N= 25 days

Example three Amoxicillin 250 mg/5 ml suspension Sig: ii tsp tid for 14 days What is the total quantity to dispense? Q= ii tsp Tsp=teaspoon so ii tsp is 2 teaspoons 2 teaspoons * (5 ml /one teaspoon)= 10 ml F=tid or three times a day, N=14 days Q*F*N= 10*3*14 420 ml

Example Four Phoslo® Sig: 1,334 mg tid ac with 6 refills What is the total quantity needed to fill this Rx for one month? For the full 7 months? Q= 2 (1,334 mg * (1 tab/667 mg)= 2) F= tid is 3 times a day N=30 days T= 2*3*30=180 tablets T for 7 months = 180 * 7= 1,260 tablets

Pediatric Dosing Pediatric dosing includes dosing for neonates, infants and children up to about 12 years of age Neonates and infants metabolize drugs differently that adults often slower in neonates. Kidney function and renally cleared drugs are removed more slowly Drugs applied topically on the skin are absorbed much better on neonates and infants than adults because of increased surface area and thinner stratum corneum Caution must always be exercised in these circumstances

Drugs that dangerous in children younger than 2 years old Potential Fatal Dose Benzocaine (Ora Base or Ora Gel) 2 ml of 10% ointment Camphor (Icy Hot) 5 ml of 20% oil Chlorpromazine (Thorazine ®) One 200 mg tablet Codeine Three 60 mg tablets Sulfonylurea Two 5 mg tablets of Glyburide Imipramine One 150 mg tablet Iron Tablets (sweet flavored) 10 tablets of 325 mg Theophylline One 500 mg tablet Verapamil One 240 mg tablet

Pediatric Dosing Formulas Fried’s Rule: PD (pediatric dose)=Age (months)/150X adult’s dose Young’s Rule : PD= Age (years)/(age (years)+12)Xadult dose Clark’s rule: PD= weight (lbs)/150Xadult dose