Numeracy: Doses, Units & Oral Medicines

A prescription reads 0.5 g of paracetamol. How many milligrams is this?

• A5,000 mg
• B50 mg
• C500 mg✓
• D5 mg

Answer: There are 1,000 milligrams in a gram, so 0.5 g × 1,000 = 500 mg. Moving the decimal point the wrong number of places gives the other figures, and a tenfold error in a dose is a serious medication error.

Digoxin 250 micrograms is prescribed. What is this dose in milligrams?

• A2.5 mg
• B25 mg
• C0.025 mg
• D0.25 mg✓

Answer: There are 1,000 micrograms in a milligram, so 250 micrograms = 250 ÷ 1,000 = 0.25 mg. Digoxin is a drug where a tenfold error is dangerous, which is why the dose is prescribed in micrograms rather than as a decimal fraction of a milligram.

A patient is prescribed amoxicillin 1 g orally. The stock is 250 mg capsules. How many capsules should be given?

• A2 capsules
• B3 capsules
• C4 capsules✓
• D8 capsules

Answer: Convert to the same units first: 1 g = 1,000 mg. Then 1,000 mg ÷ 250 mg per capsule = 4 capsules.

Paracetamol suspension contains 250 mg in 5 mL. A child is prescribed 120 mg. What volume should be given?

• A2.4 mL✓
• B1.2 mL
• C2.5 mL
• D4.8 mL

Answer: Volume = (dose required ÷ dose in stock) × stock volume = (120 ÷ 250) × 5 = 0.48 × 5 = 2.4 mL. Using 10 mL as the stock volume doubles the answer, and 2.5 mL would be 125 mg.

A medicine is prescribed at 15 mg per kg. The patient weighs 68 kg. What is the dose?

• A920 mg
• B1,200 mg
• C1,080 mg
• D1,020 mg✓

Answer: Weight-based dosing multiplies the dose per kilogram by the patient's weight: 15 × 68 = 1,020 mg.

Furosemide 100 mg is prescribed. Only 20 mg tablets are in stock. How many tablets are needed?

• A10 tablets
• B4 tablets
• C5 tablets✓
• D2 tablets

Answer: 100 mg ÷ 20 mg per tablet = 5 tablets. Always calculate from the strength actually in stock rather than the strength the dose was written for.

A patient is prescribed 500 mg of a medicine every 6 hours. What is the total daily dose?

• A500 mg
• B2,000 mg✓
• C3,000 mg
• D1,500 mg

Answer: Every 6 hours means 24 ÷ 6 = 4 doses in a day, so the daily total is 4 × 500 mg = 2,000 mg (2 g). Checking the daily total against the maximum in the BNF is part of a safe medicines check.

A 2% w/v solution contains how many milligrams of drug in 1 mL?

• A20 mg✓
• B2 mg
• C0.2 mg
• D200 mg

Answer: A percentage weight-in-volume solution gives grams per 100 mL: 2% w/v is 2 g in 100 mL, which is 2,000 mg in 100 mL, so 1 mL contains 20 mg.

A tablet contains 0.125 mg of a drug. How many micrograms is this?

• A125 micrograms✓
• B12.5 micrograms
• C1,250 micrograms
• D1.25 micrograms

Answer: 1 mg = 1,000 micrograms, so 0.125 mg × 1,000 = 125 micrograms. Digoxin is commonly prescribed at exactly this dose, which is why the conversion matters.

A child is prescribed 62.5 mg of an antibiotic. The suspension contains 125 mg in 5 mL. What volume should be given?

• A2.5 mL✓
• B0.5 mL
• C5 mL
• D1.25 mL

Answer: Volume = (62.5 ÷ 125) × 5 = 0.5 × 5 = 2.5 mL. The dose is exactly half the strength in 5 mL, so half of 5 mL is given.

A patient has drunk 1.5 litres of fluid. How many millilitres is this?

• A15,000 mL
• B1,050 mL
• C150 mL
• D1,500 mL✓

Answer: 1 litre = 1,000 mL, so 1.5 litres = 1,500 mL. Fluid charts are kept in millilitres, so conversions like this are done many times a shift.

A patient is prescribed 20 mg of a medicine four times a day. What is the total daily dose?

• A40 mg
• B80 mg✓
• C60 mg
• D100 mg

Answer: Four doses of 20 mg is 4 × 20 = 80 mg in 24 hours. Checking the daily total against the BNF maximum is part of a safe medicines round.

Paracetamol 750 mg is prescribed. The stock is 250 mg tablets. How many tablets are given?

• A4 tablets
• B2 tablets
• C3 tablets✓
• D1.5 tablets

Answer: 750 ÷ 250 = 3 tablets. With 500 mg tablets the dose would be 1.5 tablets, which is why calculating from the strength actually in the cupboard matters.

A medicine is available as 50 mg in 2 mL. The prescription is for 100 mg. What volume is drawn up?

• A4 mL✓
• B8 mL
• C1 mL
• D2 mL

Answer: Volume = (100 ÷ 50) × 2 = 2 × 2 = 4 mL. The dose is twice the amount in the stock volume, so twice the stock volume is needed.