The choice of essential drugs depends on many factors, such as the pattern of prevalent diseases; the treatment facilities; the training and experience of the available personnel; the financial resources; and genetic, demographic and environmental factors.

Because of differing views on the definition of an essential drug in terms of what is meant by the “health care needs of the majority” of the population, the model list has been gradually expanded since its introduction. Some drugs are included that are essential only if a therapeutic programme is planned to address the diseases for which these drugs are used. For example, the cytotoxic drugs (section 8.2 of the model list) are essential only if a comprehensive cancer treatment programme is planned. Such a programme requires adequate hospital, diagnostic and clinical laboratory facilities for its implementation.

In contrast, the drugs used in palliative care (section 8.4) are always essential, even when a comprehensive cancer treatment programme does not exist.

The selection of essential drugs must always be based on valid scientific evidence. Only those drugs should be selected for which sound and adequate data on efficacy and safety are available from clinical studies.

Each selected drug must be available in a form in which adequate quality, including bioavailability, can be assured; its stability under the anticipated conditions of storage and use must be established.

Where two or more drugs appear to be similar in the above respects, the choice between them should be made on the basis of a careful evaluation of their relative efficacy, safety, quality, price and availability.

In cost comparisons between drugs, the cost of the total treatment, and not only the unit cost of the drug, must be considered. The cost/benefit ratio is a major consideration in the choice of some drugs for the list. In some cases the choice may also be influenced by other factors, such as comparative pharmacokinetic properties, or by local considerations such as the availability of facilities for manufacture or storage.

Most essential drugs should be formulated as single compounds. Fixed-ratio combination products are acceptable only when the dosage of each ingredient meets the requirements of a defined population group and when the combination has a proven advantage over single compounds administered separately in therapeutic effect, safety or compliance.

At its previous meetings, the Committee did not always require a review of all the available evidence. For example, recommendations by WHO programmes were often accepted based on the assurance of the programme that appropriate evidence had been received and was considered to be adequate. At its present meeting, the Committee required that a summary of the appropriate evidence be presented for review. When such evidence was not available, action on a request was deferred or the proposed change was rejected, with a request that supporting evidence be presented with the proposed change at the next meeting of the Committee. The urgency of some of the proposals was such that action was required at this meeting even though a systematic review of the evidence was unavailable. For these proposals, action was taken based on the information that was available and the best judgement of the Committee.