PHARMACEUTICAL CALCULATIONS III
The therapeutic dose of a medication is based, generally, on its having a desired
concentration in the patient's body. What is considered to be an ideal dose for an
adult, when placed in the body of a small child, would give a concentration that is much
greater than desired and could cause an adverse effect. Therefore, it is normally
necessary to calculate a smaller dose for the child. This lesson will explain the
methods used in pharmacy to calculate patient dosages.
3-2. WHAT IS A DOSE?
a. The term DOSE refers to the amount of medication that a patient must take
at one time to produce the optimum therapeutic effect. The terms "average dose,"
"usual dose," and "adult dose" are based on the amount of medication needed to treat
the average size adult (150-154 lbs.) with optimum effect. In order to make it easier to
calculate dosages for other than average size patients, many drug manufacturers have
established recommended doses based on the patient's weight or Body Surface Area
(BSA). With this type of recommended dose, it is easy to calculate a dose for any size
person using the same formula.
Examples of recommended doses:
(1) Pyrantel pamoate (Antiminth) has a recommended dose of 11 mg/kg of
body weight and is given in a single dose.
(2) Isoniazid (INH) has a recommended dose of 450 mg/M2/24 hours. The
M2 refers to one square meter of body surface area.
(3) Meperidine hydrochloride (Demerol) has a recommended dose of 6
mg/kg/24 hours for pain and is given in divided doses four to six times daily. No single
dose should exceed 100 mg.
3-3. PROBLEMS USING MANUFACTURER'S RECOMMENDED DOSE
Solution by ratio and proportion: (Example problems)
a. Cyclophoshamide (Cytoxan) 50 mg tablets are used in the treatment of
neoplasms (abnormal growths or tumors) and have a recommended dose of 5 mg/kg of
body weight to be given in a single dose daily. How many tablets should be dispensed
to a 110-lb patient as a ten-day regimen? (NOTE: A regimen is a treatment plan.).