Common Registration Assessment, Part 1
Doxycycline capsule strengths and course totals for the GPhC assessment
Three original practice questions, with the working shown. Read the stated prescription or plan before calculating.
Published 19 October 2026. Independent revision material, not GPhC questions, not for patient care.
The BNF lists doxycycline capsules in two strengths: 50 mg and 100 mg.
The BNF gives a common adult pattern as 200 mg on the first day, then 100 mg once a day for the rest of the course.
Each question fixes the values needed for its arithmetic. Write the unit beside every step and keep the first day separate from the rest of the course. All prescriptions and patients here are fictional; these are revision materials, not for patient care.
The method
- Identify the exact strength, dose and schedule supplied in the question.
- Split a first-day dose from the daily doses that follow when the schedule changes.
- Calculate capsules per day; divide prescribed mg by mg per capsule.
- Add the first day to the remaining days only when a course total is requested.
- Check the capsule count is a whole number before reporting it.
Three practice questions
Work each one on paper first, then open the answer. All patients and prescriptions are fictional.
Question 1
A fictional prescription specifies doxycycline 200 mg on day 1, then 100 mg once a day for 6 more days, using 100 mg capsules. How many capsules is the course?
Show the answer
Answer: 8 capsules
Working: Day 1: 200 ÷ 100 = 2 capsules. Then 100 ÷ 100 = 1 capsule a day for 6 days = 6 capsules. 2 + 6 = 8 capsules.
Question 2
A fictional prescription specifies doxycycline 100 mg once a day for 7 days using 50 mg capsules. How many capsules is the course?
Show the answer
Answer: 14 capsules
Working: 100 ÷ 50 = 2 capsules a day. 2 × 7 = 14 capsules.
Question 3
A fictional prescription specifies doxycycline 200 mg on day 1, then 100 mg once a day for 13 more days, using 100 mg capsules. How many capsules is the 14 day course?
Show the answer
Answer: 15 capsules
Working: Day 1: 200 ÷ 100 = 2 capsules. Then 1 capsule a day for 13 days = 13 capsules. 2 + 13 = 15 capsules.
Exam approach
A changing schedule means two calculations: the first day, then the rest. Write both lines before adding.
Where marks are lost
- Applying the first-day dose to every day of the course.
- Forgetting to add the first day into a course total.
- Using 100 mg per capsule when the stated strength is 50 mg.
Frequently asked questions
Are these official assessment questions?
No. They are original revision exercises, not official GPhC questions and not for patient care.
Do I need to memorise the strengths?
The exercise supplies them. In the assessment, read any given product information exactly as written.
How should I check the answer?
Check the requested unit, then work backwards from the quantity to the stated dose and duration.
Sources
The questions above are original and use fictional patients and prescriptions. PreRegExamPrep is not affiliated with or endorsed by the General Pharmaceutical Council.
Practise until the method is automatic
Try 15 free questions with worked answers. No sign-up required.
More calculation topics
- Dose by weight calculations for the GPhC assessment
- Volume to give: liquid medicine calculations
- Percentage strength calculations: w/v, w/w and ratios
- Dilution calculations with C1V1 = C2V2
- Infusion rate calculations in mL per hour
- Drip rate calculations: drops per minute
- Body surface area calculations
- Creatinine clearance (Cockcroft-Gault) calculations
- Moles and millimoles calculations
- Displacement volume calculations for reconstitution
- Quantity to supply calculations
- Using a provided formula in calculations
- Dose information from packaging and labels
- Diluting a stock to a lower strength
- Medicine cost and switch calculations
- Rounding rules in calculations
- Enteral feed rate calculations
- Electrolyte content over time
- Infusion dose per kg per hour
- Rounding doses to measurable volumes
- Relative risk and relative risk reduction
- Absolute risk reduction and number needed to treat
- Odds ratio and confidence intervals
- Relative versus absolute risk reduction
- Loading dose and volume of distribution
- Top-up doses from measured drug levels
- Salt correction factors
- Mass and molar drug concentrations
- Half-life and falling drug levels
- Steady-state concentration and infusion rate
- Bioavailability and equivalent oral doses
- Oral maintenance dose and dosing interval
- Elimination rate constant and half-life
- Accumulation towards steady state
- Ratio strengths and one in X
- Serial dilution factors
- Mixing solutions of different strengths
- Tablet quantities for reducing regimens
- Equivalent doses from provided conversion tables
- Thresholds and dose-banding algorithms
- Unit conversions for mass and volume
- Units and units per mL
- Reconstituting powders and final concentration
- Millimoles and milliequivalents
- Infusion time and volume remaining
- Days supply from pack sizes
- Percentage w/v and w/w conversions
- Ideal and adjusted body weight from formulae
- Diluting creams and ointments by weight
- Scaling a formula to a batch size
- Age-band doses from a provided table
- Tablet fractions and split doses
- Stones, pounds, inches and metric conversions
- Micrograms per minute from mg per hour
- Percentage change and mark-up
- Molar solutions and mass per volume
- Osmolarity from component concentrations
- Direct proportion and cross-multiplication
- Scientific notation and very small quantities
- Stock use and reorder quantities
- Parts per million and very dilute solutions
- Sodium chloride equivalents and isotonic solutions
- Density and specific gravity conversions
- Doses per day and hours between doses
- Finding the volume of diluent to add
- Capping a dose at a stated maximum
- Whole vials and ampoules to draw up
- Reading values from a provided graph
- Micrograms per kg per minute infusions
- Percentage composition of a compound preparation
- Suppository displacement values
- Isotonicity by freezing point depression
- Energy content of parenteral nutrition
- Syringe driver rate calculations
- Area under the curve calculations
- Renal dose adjustments using provided bands
- Insulin units and infusion calculations
- Working backwards from a stated dose
- Doses and days from multi-dose devices
- Combined additives in one infusion
- BMI calculations for the GPhC assessment
- Fewest tablets to make a dose
- Clearance from half-life and volume of distribution
- Maintenance fluid requirements by weight
- Sodium load from effervescent tablets
- Ethanol content of liquid preparations
- Oxygen cylinder duration calculations
- Urine output and fluid balance calculations
- Quantities for titration packs
- Infusion line priming and dead space
- Alcohol unit calculations for the GPhC assessment
- Paracetamol syrup calculations for children
- Ibuprofen liquid calculations for children
- Vitamin D conversions for the GPhC assessment
- Ferrous sulfate drops and tablet calculations
- Amoxicillin liquid and capsule calculations
- Folic acid dose and quantity calculations
- Adult paracetamol and the daily maximum
- Calcium and iron intake calculations
- Salt limit calculations by age
- Fingertip unit calculations for the GPhC assessment
- Adrenaline dose calculations for anaphylaxis
- Breakthrough opioid dose calculations
- IV fluid bolus calculations in children
- Neonatal maintenance fluid calculations
- Enoxaparin dose calculations by weight
- Levothyroxine dose by weight calculations
- Methotrexate weekly dose calculations
- Sensitivity, specificity and predictive value calculations
- Hypertonic saline bolus calculations in children
- Prednisolone tablet and liquid calculations for the GPhC assessment
- Cetirizine liquid volume and supply calculations for the GPhC assessment
- Lactulose dose and bottle supply calculations for the GPhC assessment
- Metformin tablet and liquid strength calculations for the GPhC assessment
- Flucloxacillin capsule and course supply calculations for the GPhC assessment
- Clarithromycin liquid strength and formulation calculations for the GPhC assessment
- Senna tablet and syrup quantity calculations for the GPhC assessment
- Nitrofurantoin formulation and course quantity calculations for the GPhC assessment
- Aciclovir five-times-daily course calculations for the GPhC assessment
- Oral rehydration fluid deficit calculations for the GPhC assessment
- Warfarin tablet strengths and daily doses for the GPhC assessment
- Digoxin microgram tablets and daily doses for the GPhC assessment
- Co-amoxiclav tablet courses and liquid shelf life for the GPhC assessment
- Phenoxymethylpenicillin age band doses and liquid strengths for the GPhC assessment
- Cefalexin capsule counts and course supply for the GPhC assessment
- Co-codamol strengths and 24-hour maximums for the GPhC assessment
- Trimethoprim tablets, liquid and course lengths for the GPhC assessment
- Macrogol sachet schedules and water volumes for the GPhC assessment
- Colchicine tablet counts and course maximums for the GPhC assessment
- Furosemide tablet and liquid strengths for the GPhC assessment
- Amitriptyline tablet strengths and daily doses for the GPhC assessment
- Ramipril titration packs and tablet strengths for the GPhC assessment
- Amlodipine tablet and liquid strengths for the GPhC assessment
- Atorvastatin tablet strengths and daily doses for the GPhC assessment
- Citalopram drops and tablet equivalents for the GPhC assessment
- Fexofenadine tablet strengths by age for the GPhC assessment
- Gliclazide standard and slow-release tablets for the GPhC assessment
- Lansoprazole capsule strengths and course lengths for the GPhC assessment
- Naproxen tablet strengths and daily maximums for the GPhC assessment
- Propranolol tablet strengths and daily doses for the GPhC assessment
- Fluoxetine capsule strengths and liquid doses for the GPhC assessment
- Simvastatin tablet strengths and evening doses for the GPhC assessment
- Losartan tablet strengths and daily doses for the GPhC assessment
- Gabapentin capsules, tablets and three daily doses for the GPhC assessment
- Montelukast tablets, chewables and granules by age for the GPhC assessment
- Salbutamol inhaler puffs and daily maximums for the GPhC assessment
- Mirtazapine tablets, liquid and bedtime doses for the GPhC assessment
- Allopurinol tablet strengths and daily doses for the GPhC assessment
- Spironolactone tablet strengths by condition for the GPhC assessment
- Doxazosin standard and slow-release tablets for the GPhC assessment
- Omeprazole capsule strengths and daily doses for the GPhC assessment
- Sertraline tablet strengths and dose steps for the GPhC assessment
- Bisoprolol tablet strengths and daily doses for the GPhC assessment
- Pregabalin capsule strengths and daily doses for the GPhC assessment
- Tramadol capsule strengths and daily maximums for the GPhC assessment
- Venlafaxine tablet strengths and daily doses for the GPhC assessment
- Clopidogrel tablet strengths and supply counts for the GPhC assessment
- Bendroflumethiazide tablet strengths and daily doses for the GPhC assessment
- Tamsulosin capsule strengths and daily doses for the GPhC assessment
See also the formula sheet, the eight sample questions and the approved calculator page.