United States
Hong KongStrengths and duration
Strength describes how much active ingredient a preparation contains, expressed in several notations. Duration connects to days supply, which is a calculation.
A knowledge area that looks like pharmacology and is answered like arithmetic.
How strength is expressed
- A mass per unit: 500 mg per tablet
- A mass per volume: 250 mg per 5 mL
- A percentage, which for a liquid is grams per 100 mL
- A ratio strength, such as 1 to 1000
- Units, for preparations measured biologically
Converting between the notations is where the marks are, and it is the same skill as the percentage and ratio strength calculations. Run it backwards.
The conversion worth having automatic
A percentage times ten gives mg per mL. Five percent is 50 mg per mL.
That single relationship unlocks most strength questions and most dilution questions at once.
A bottle labeled 250 mg per 5 mL is not a 250 mg bottle. Questions exploit the difference, and the distractor for reading the concentration as the total is reliably present. Check what the number is attached to before using it.
Duration of therapy
How long a course is intended to run. It connects directly to days supply: total quantity divided by quantity used per day.
A question giving you a duration and asking for a quantity is the same calculation run backwards.
Why it matters in practice
Days supply drives refill timing and insurance claims, so an error here has consequences beyond the exam. That is why it is examined rather than assumed. Check what it is attached to.
Units as a special case
Some preparations are measured in units rather than by mass, and the units per mL on the container is what a calculation needs. That figure is on the packaging rather than in the Sig. Multiply by ten.
Common questions
How is drug strength expressed?
Mass per unit, mass per volume, percentage, ratio strength, or units.
How do I convert a percentage?
Multiply by ten for mg per mL. Five percent is 50 mg per mL.
What is the common misreading?
Treating a concentration such as 250 mg per 5 mL as the total in the container.
How does duration connect to calculations?
Through days supply - total quantity divided by quantity used per day.
What about preparations measured in units?
The units per mL is on the packaging, not in the Sig, and it is what the calculation needs.