Oral Liquid Medicine Calculations: NMC CBT Practice

Liquid questions are tablet questions with one extra term. Where a tablet answer is a count, a liquid answer is a volume, so the stock’s own volume joins the formula.

Volume to give = (dose prescribed ÷ dose in stock) × volume the stock dose is in.

Or in ward language: what you want, over what you’ve got, times what it’s in.

Worked example. Prescription: amoxicillin 250 mg. Stock: 125 mg/5 mL suspension.

• Ratio: 250 ÷ 125 = 2.
• Volume: 2 × 5 = 10 mL.

The “per 5 mL” convention exists because 5 mL is a standard medicine spoon, so most UK oral syrups and suspensions are labelled that way. But not all of them. Some oral solutions are per 1 mL; injectables usually are. The first thing to read on any liquid question is the “per what”.

The error the exam is fishing for

Nearly every wrong option on a liquid question comes from one of two mistakes.

The first is stopping at the ratio. 250 ÷ 125 = 2, and a tired candidate writes 2 mL. The ratio isn’t a volume until you’ve multiplied by the stock volume. If the strength says per 5 mL and your answer isn’t a multiple or clean fraction of 5, pause.

The second is the familiar one: unit mismatch. A prescription in grams against a syrup in milligrams. Convert before you touch the formula. The unit conversions chapter drills this if you skipped it.

What realistic answers look like

Oral liquid answers land between about 1 mL and 20 mL for adult single doses, and awkward-looking volumes are normal. 7.5 mL, 3.75 mL, 12.5 mL: all fine, all measurable with an oral syringe. Don’t round to the nearest spoonful, and don’t distrust a decimal. Distrust an answer of 100 mL. Nobody drinks half a glass of amoxicillin.

One more habit that pays in the exam room: after calculating, restate the answer against the stock. “10 mL of a 125 mg/5 mL suspension is two lots of 125 mg, which is 250 mg. Matches the prescription.” That reverse check takes ten seconds and catches almost everything.

The same formula, with the same traps, runs the injection volume questions in the next chapter; only the stock volumes shrink. Five practice questions first.