Common Registration Assessment, Part 1

Olopatadine eye drop counts and bottle supply for the GPhC assessment

Three original practice questions, with the working shown. Read the stated prescription or plan before calculating.

Published 10 November 2026. Independent revision material, not GPhC questions, not for patient care.

An unlabelled eye-drop bottle with its cap beside it
Photo from Pexels, used under the Pexels licence. It is a stock image and does not show the exercise.

The manufacturer's UK product information for Opatanol 1 mg/ml eye drops states that the dose is one drop in the conjunctival sac of the affected eye or eyes twice daily (8 hourly), that treatment may be maintained for up to four months if considered necessary, and that the drops may be used in children aged three years and older at the same dose as adults.

The same information states that the drops are supplied in 5 ml bottles, in cartons containing 1 or 3 bottles, and that when other eye medicines are used at the same time an interval of five minutes should be left between applications, with eye ointments last.

A 1 mg/ml strength means each ml contains 1 mg of olopatadine. Drop-volume questions on this page use the working assumption of 20 drops per ml, stated with the question, because the label does not print a drop volume.

Each question fixes the values needed for its arithmetic. Write the unit beside every step and distinguish drops per day from days of supply. All prescriptions and patients here are fictional; these are revision materials, not for patient care.

The method

  1. Multiply one drop by the stated number of affected eyes, then by two times daily, for the daily drop count.
  2. Multiply the daily drop count by the stated days for a course total.
  3. Using the stated 20 drops per ml assumption, divide the bottle's total drops by the daily drop count for the days of supply.
  4. Keep drops, ml, bottles and days separate; convert only when the question asks.

Three practice questions

Work each one on paper first, then open the answer. All patients and prescriptions are fictional.

Question 1

A fictional patient uses olopatadine eye drops in both eyes at the label dose. How many drops are used each day?

Show the answer

Answer: 4 drops

Working: 1 drop x 2 eyes = 2 drops per administration. 2 drops x 2 times daily = 4 drops daily.

Question 2

A fictional patient uses one drop in both eyes twice daily for 28 days. How many drops are used in total?

Show the answer

Answer: 112 drops

Working: 4 drops daily x 28 days = 112 drops.

Question 3

A fictional patient uses one drop in both eyes twice daily (4 drops daily). Using the working assumption of 20 drops per ml, how many days does one 5 ml bottle last, and how many days does the 3-bottle carton last?

Show the answer

Answer: 25 days; 75 days

Working: 5 ml x 20 drops per ml = 100 drops per bottle. 100 / 4 = 25 days. Three bottles: 300 / 4 = 75 days, within the label's four-month maximum duration.

eMC SmPC: OPATANOL 1mg/ml eye drops solution

https://www.medicines.org.uk/emc/product/6768/smpc

Where marks are lost

  • Counting drops for one eye when the question states both eyes are treated.
  • Treating the 20 drops per ml figure as a label value; it is a working assumption stated in the question.
  • Dividing the bottle's drops by drops per administration instead of drops per day, which doubles the apparent supply.

Frequently asked questions

Are these official assessment questions?

No. They are original revision exercises, not official GPhC questions and not for patient care.

Does this calculation decide a real olopatadine supply?

No. Follow the stated regimen in the exercise. Real use follows the label, with a five-minute gap when other eye medicines are used at the same time.

How should I check the answer?

Multiply the drops per administration by the stated eyes and by the two daily doses, then divide the bottle's total drops by the daily count.

Sources

The questions above are original and use fictional patients and prescriptions. PreRegExamPrep is not affiliated with or endorsed by the General Pharmaceutical Council.

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More calculation topics

See also the formula sheet, the eight sample questions and the approved calculator page.