Pharmaceutical Sciences and Calculations
What to know
Expect questions on dose calculation, concentration and dilution, unit conversion, and pharmaceutical formulation. Master the core relationships: concentration equals amount divided by volume; amount equals concentration times volume; dilution follows the principle that the amount of drug is conserved (C1 × V1 = C2 × V2). Know how to convert between grams, milligrams, micrograms, and nanograms, and between millilitres and litres, and be fluent with percentage concentrations: % w/v means grams per 100 mL, % w/w means grams per 100 g, and % v/v means millilitres per 100 mL.
Understand body-weight and body-surface-area dosing, and paediatric dose estimation. Know the ideas behind bioavailability, bioequivalence, and dissolution testing. Be familiar with dosage form design: tablets, capsules, sustained-release systems, suppositories, and sterile products. Understand isotonicity and osmolarity in a basic way, and the rationale for buffers, preservatives, and antioxidants in formulations.
Traps
- Mixing up w/v, w/w, and v/v percentages.
- Forgetting to convert units before calculating (mg versus g is a factor of 1000).
- Treating a dilution as if the volume stayed the same.
- Reporting an answer with implausible precision or the wrong unit.
- Ignoring the difference between the labelled salt form and the active base.
Worked example / rule of thumb
Dilution rule: drug amount before equals drug amount after. A 20 mL vial of 25% w/v contains 25 g per 100 mL, so 20 mL holds 5 g. Dilute it to 250 mL and the new concentration is 5 g ÷ 250 mL = 2% w/v, or 20 mg/mL. Always sanity-check: dilution must lower the concentration and raise the volume while the total drug stays fixed.