Common Registration Assessment, Part 1
Sodium load from effervescent tablets
Three original questions on sodium in soluble medicines, with the working shown and the usual traps pointed out.
Published 4 October 2026. Independent revision material, not GPhC questions, not for patient care.
Effervescent and soluble tablets can carry a heavy sodium load. The stem usually gives sodium in millimoles per tablet, so convert to milligrams: 1 mmol of sodium is 23 mg.
To express the load as salt, multiply sodium by 2.5: the NHS notes that 2.4 g of sodium is about 6 g of salt, the adult daily maximum.
The method
- Note the sodium content per tablet in mmol and the number of tablets per day.
- Multiply mmol by 23 to get mg of sodium per tablet.
- Multiply by tablets per day for the daily sodium in mg, then convert to grams.
- Multiply grams of sodium by 2.5 for the salt equivalent.
- Compare with the 6 g adult daily salt maximum.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
An effervescent tablet contains 18 mmol of sodium. A patient takes 2 tablets twice daily. What is the daily sodium intake from the tablets, and the salt equivalent?
Show the answer
Answer: About 1.66 g of sodium, about 4.1 g of salt
Working: 4 tablets × 18 mmol = 72 mmol. 72 × 23 mg = 1,656 mg = 1.66 g sodium. 1.66 × 2.5 = 4.1 g of salt.
Question 2
A soluble analgesic tablet contains 18.6 mmol of sodium. The maximum dose is 8 tablets daily. What is the maximum daily sodium load as salt, and how does it compare with the 6 g adult limit?
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Answer: About 8.6 g of salt, above the 6 g limit on its own
Working: 8 × 18.6 = 148.8 mmol. 148.8 × 23 = 3,422 mg = 3.42 g sodium. 3.42 × 2.5 = 8.6 g salt, more than the 6 g daily maximum before any food.
Question 3
A patient takes 2 effervescent tablets twice daily, each containing 16 mmol of sodium, for 7 days. What is the total salt equivalent over the week to one decimal place?
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Answer: About 25.8 g of salt
Working: 4 × 16 = 64 mmol per day. 64 × 23 = 1,472 mg = 1.47 g sodium per day. Over 7 days: 10.3 g sodium. 10.3 × 2.5 = 25.8 g salt.
How this is examined
Keep mmol and mg separate in your working. The common error is multiplying tablets by 23 directly when the stem gives mmol per tablet.
Where marks are lost
- Forgetting that mmol of sodium converts at 23 mg per mmol, not 58.5 (that is sodium chloride).
- Counting the dose frequency wrong: twice daily with 2 tablets is 4 tablets, not 2.
- Reporting sodium when the question asks for salt: the two differ by a factor of about 2.5.
Frequently asked questions
Why do soluble tablets contain so much sodium?
The fizz comes from sodium bicarbonate and sodium carbonate, which release carbon dioxide in water. The sodium is built into the formulation.
What is the adult daily salt maximum?
The NHS advises adults to have no more than 6 g of salt a day, which is about 2.4 g of sodium.
Sources
The questions above are original and use fictional drugs. 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
- 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
See also the formula sheet, the eight sample questions and the approved calculator page.