1.9: Dosage Calculations I
- Calculate drug dosages using conversion factors.
To Your Health: Dosages
A medicine can be more harmful than helpful if it is not taken in the proper dosage. A dosage (or dose) is the specific amount of a medicine that is known to be therapeutic for an ailment in a patient of a certain size. Dosages of the active ingredient in medications are usually described by units of mass, typically grams or milligrams, and generally are equated with a number of capsules or teaspoonfuls to be swallowed or injected. The amount of the active ingredient in a medicine is carefully controlled so that the proper number of pills or spoonfuls contains the proper dose.
Most drugs must be taken in just the right amount. If too little is taken, the desired effects will not occur (or will not occur fast enough for comfort); if too much is taken, there may be potential side effects that are worse than the original ailment. Some drugs are available in multiple dosages. For example, tablets of the medication levothyroxine sodium, a synthetic thyroid hormone for those suffering from decreased thyroid gland function are available in 11 different doses, ranging from 25 micrograms (mcg) to 300 mcg. It is a doctor’s responsibility to prescribe the correct dosage for a patient; it is a pharmacist’s responsibility to provide the patient with the correct medicine at the dosage prescribed; and it is the nurse's or caregivers responsibility to check and provide the patient with the medication. Thus, proper quantities—which are expressed using numbers and their associated units—are crucial for keeping us healthy.
Drug dosage calculations
Drug dosage calculation is required if the physician’s order is different from what is available. The medication may be administered as fluid or tablet.
Epinephrine is used to treat peanut allergies. A 5 year old patient needs an immediate treatment of 0.12 mg of epinephrine. The solution is available in concentration of 0.50 mg/ml. Calculate the volume in ml required for the patient.
Solution
The first step is to understand the meaning of concentration of solution. There are 0.50 mg in 1 mL of solution. From this relationship, we can construct the conversion factor. We use the conversion factor that will cancel out the original unit, mg , and introduce the unit we are converting to, which is mL .
\[ 0.12 \cancel{\rm{mg}} \times \dfrac{1\; \rm{mL}}{0.50\; \cancel{\rm{mg}}} = 0.24\; \rm{mL} \nonumber \]
Thus the patient requires 0.24 mL of epinephrine.
A physician ordered 100 mg of Demerol. Demerol is available as 50 mg per tablet. How many tablets should the nurse administer?
Solution
We start with the given, 100 mg. We want to change the unit from mg to tablets . There are 50 mg in 1 tablet (Remember that per tablet means one tablet. From this relationship, we can construct the conversion factor. We use the conversion factor that will cancel out the original unit, mg , and introduce the unit we are converting to, which is tablet .
\[ 100 \cancel{\rm{mg}} \times \dfrac{1\; \rm{tablet}}{50\; \cancel{\rm{mg}}} = 2\; \rm{tablets} \nonumber \]
Hence, the nurse should administer 2 tablets.
The physician ordered 20 mg of Valium. Valium is available as 10 mg per tablet. How many tablets should the nurse administer?
Solution
Start with 20 mg of Valium. For the conversion factor, we know that 10 mg Valium = 1 tablet
\[ 20 \cancel{\rm{mg}} \times \dfrac{1\; \rm{tablet}}{10\; \cancel{\rm{mg}}} = 2\; \rm{tablets} \nonumber \]
Contributors and Attributions
- Wikipedia
- Leticia Colmenares (UH-WCC)
- Deboleena Roy (American River College)