Good prescribing decisions take into account the patient's wishes and needs, as well as evidence on a medicine's clinical effectiveness, safety and cost effectiveness, and identifies a therapeutic outcome. This complex process requires balancing potentially conflicting pieces of information, and needs to adapt to changes in patients' requirements, evidence and costs. For GPs, the computerised clinical system exists to aid prescribing by maintaining a record of the patient's present and past prescriptions, as well as providing crucial advice and warnings about drug doses and interactions. But what should happen when clinical systems do not reflect local formulary choices and guidelines?

Increasingly, prescribing support software (e.g. ScriptSwitch) is being commissioned and programmed by primary care trusts (PCTs). Such software links to GP clinical systems, to provide prescribers with local formulary choices and advice on the latest cost saving, safety and effectiveness issues relating to medicines. At the point of prescribing, the software will offer alternative prescribing options that are usually cheaper than the one initially selected, or messages reminding clinicians of any relevant information; it also calculates the prescribing cost of rejecting or accepting the changes. PCTs can monitor uptake of the prompts and their financial impact to help refine the information provided. As the NHS targets significant efficiency savings over the next few years, this support software could have an important role in offering cost effective prescribing choices and keeping prescribers updated and engaged with local decision-making. However, it does not replace the need to exercise clinical judgement and reject recommendations not in the best interests of patients. Blanket prescribing changes (e.g. transferring all patients from one drug to another) could be harmful. The software should be regularly updated and used to provide a balance between improving clinical and financial outcomes. Like satellite navigation systems it should not be followed unquestioningly.

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