How to write a safe prescription

What are the elements of a prescription?

Every prescription must let someone else give the right drug, in the right amount, by the right route, at the right time. Each missing element is a point where an error can occur.

1. Drug name, written generically.
2. Dose, with a clear unit.
3. Route.
4. Frequency, or the instruction that it is once only.
5. Indication and maximum dose, if as required.
6. Duration or stop date, where relevant.
7. Signature and date, as the chart requires.

In the PSA the written part of a prescribing item is marked separately from the drug choice, so the written elements carry marks.

How should I write names and units?

Use the generic name and write units in full where the BNF advises. Brand names can lead to duplication, and unclear units lead to tenfold and thousandfold errors.

• Use generic names, for example paracetamol rather than a brand.
• Write micrograms, nanograms and units in full, not abbreviated.
• Write 500 mg rather than 0.5 g, and 100 micrograms rather than 0.1 mg.
• Where a decimal is unavoidable, use a leading zero (0.5 mL, not .5 mL) and never a trailing zero (5 mg, not 5.0 mg).
• State the dose as an amount of drug, not a volume of liquid.

Where a drug has more than one salt or formulation, and the BNF distinguishes them, name the one you intend.

How do I choose and write the route and frequency?

Choose a route the drug can actually be given by and the patient can use, and state a frequency in the chart's own words. A blank frequency is marked as wrong.

• Check the BNF medicinal forms if unsure that a route exists for the drug.
• Use standard route abbreviations where the chart accepts them, or write the route in full.
• Write the frequency clearly, for example once daily, twice daily or every set number of hours.
• Take extra care with drugs given weekly or on alternate days, and write that explicitly.

Think about the patient too: a patient who is vomiting or nil by mouth may not be able to take an oral drug.

What is the difference between regular, once-only and as-required?

The three sections of the chart are different forms and the instruction line tells you which one is wanted. Writing the right drug in the wrong form loses marks.

An as-required prescription without an indication and a maximum dose in 24 hours is incomplete. Without a limit, repeated doses can add up beyond the safe total.

When does a prescription need a duration or stop date?

Write a duration or stop date whenever the treatment is meant to be short, and a review date where it should be revisited. Open-ended courses of some drugs cause harm.

• Short courses of antibiotics should have a stated duration or review date.
• Short courses of other drugs, such as steroids for an acute problem, need a clear stop.
• Check the item's wording for whether a duration is required.

If the question states a duration, include it; if the BNF gives a standard course length for the indication, use that.

What should I check before prescribing?

Check the patient before the drug: allergies, weight, renal function, pregnancy and current medicines. These details change the choice or the dose.

• Allergy: avoid the drug and cross-reacting classes.
• Weight: use it for weight-based doses, especially in children.
• Renal function: look up the BNF renal impairment advice; some drugs need a dose change or avoidance.
• Pregnancy and breastfeeding: check the BNF for suitability.
• Other medicines: check interactions.

For renal dosing, the BNF indicates whether eGFR or creatinine clearance should guide the dose; follow what the monograph says.

What should I know about controlled drugs?

Controlled drugs have additional legal requirements for prescriptions, and the BNF has a section setting these out. At exam level, know that they exist and write the prescription with extra care.

• Expect stricter prescription-writing rules than for ordinary drugs.
• Take particular care with opioids: dose, unit, route and frequency must be exact.
• Use the BNF guidance rather than recall for any specific requirement.

Check yourself

• What are the seven elements of a prescription?
• Why do you write 0.5 mL but 5 mg?
• What two extra elements does an as-required prescription need?
• How does a once-only prescription differ from a regular one?
• When do you add a stop date?
• Which patient details do you check before choosing a dose?

Questions people ask

Related revision notes

Sources: BNF guidance on prescription writing; BNF; NICE guidance on antimicrobial prescribing. These notes are for exam revision and are not patient-specific advice; in the exam and in practice, the BNF is the authority.

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