Pharmaceutical excipients – where do we begin?

Excipients have been defined in many ways, including as inert substances used as vehicles and diluents for drugs. The problem with this definition is that in recent years excipients have proved to be anything but inert, not only possessing the ability to react with other ingredients in the formulation, but also to cause adverse and hypersensitivity reactions in patients. These range from a mild rash to a potentially life-threatening reaction. Different brands of the same drug may contain different excipients, especially preservatives and colourants. The Consumer Medicines Information provides a list of excipients, and information on the safety of individual excipients can be found in drug reference guides.

The word excipient is derived from the Latin excipere, meaning 'to except', which is simply explained as 'other than'. Pharmaceutical excipients are basically everything other than the active pharmaceutical ingredient. Ideally, excipients should be inert, however, recent reports of adverse reactions have suggested otherwise.

The best new therapeutic entity in the world is of little value without an appropriate delivery system.¹ Today, medicines are available in many dosage forms including tablets, capsules, oral liquids, topical creams and gels, transdermal patches, injectable products, implants, eye products, nasal products, inhalers and suppositories. Pharmaceutical excipients are substances that are included in a pharmaceutical dosage form not for their direct therapeutic action, but to aid the manufacturing process, to protect, support or enhance stability, or for bioavailability or patient acceptability. They may also assist in product identification and enhance the overall safety or function of the product during storage or use.²

Thousands of different excipients are used in medicines and make up, on average, about 90% of each product. They represent a market value of €3 billion (almost $4 billion) accounting for 0.5% of the total pharmaceutical market according to industry experts.³

The list of purposes for which excipients are used, as defined in international pharmacopoeias, is extremely long. Many excipients have more than one use, which can be an advantage since it reduces the number of excipients needed and minimises the risk of interactions between them.

Tablets are the most widely used dosage form. Their manufacture can be a complex process and considerable ingenuity and formulation expertise are required to produce a product that will be stable during storage, transport and handling, yet will release its active pharmaceutical ingredient as required once ingested.⁴ Various excipients are used to achieve this (Table 1).²

Ideally, an excipient is pharmacologically inactive, non-toxic, and does not interact with the active ingredients or other excipients. However, in practice few excipients meet these criteria. Toxicity may relate to compounds used as excipients in the final dosage form, or to residues of compounds (such as solvents) used during the manufacturing process.² Table 2 shows examples of adverse reactions that have occurred with excipients.

Colouring agents

Owing to their widespread and relatively large use in food, a number of colours in current use have been associated with adverse effects, although in a relatively small number of people.⁵ The role of food additives in hyperactive behaviour has been debated for many years. In 2007 a study was published linking the use of six colours (tartrazine, quinoline yellow, sunset yellow, carmoisine, ponceau 4R and allura red) with behavioural problems in children. However, after reviewing the results of the study, the European Food Standards Agency concluded that no change in legislation was needed.⁵

When presented with a patient who has an adverse reaction, it is important to be aware that reactions may not always be due to the active ingredient (Fig. 1). They are more likely to occur if the patient has an existing sensitivity to similar ingredients, or is on multiple medicines, or when the quantity of excipients may be high relative to body weight, for example in premature babies.⁶ Excipients present in their current and past medication history should also be considered. This will help to rule out which ingredients may be causing the adverse effects.

The Consumer Medicine Information leaflet, which is often readily available online (www.nps.org.au/search_by_medicine_name), will have a list of the excipients included in the medicine. This list can be found under 'Product description' and may be titled 'Other ingredients' or 'This product also contains...'

The Pharmacopoeias (US Pharmacopoeia, British Pharmacopoeia) contain monographs for many excipients. However, not all excipients reach these texts due to companies withholding data because of concerns about releasing proprietary information. The Handbook of Pharmaceutical Excipients contains monographs for 340 excipients, with each monograph including a 'Safety' section that presents adverse reactions that have been reported.⁵ Martindale² has safety information on excipients and is a required text for hospital and community pharmacists. The monographs for each excipient contain a section on adverse effects reviewed from the literature.

Medicines contain ingredients other than the active drug that are essential for their manufacture, stability and function. These ingredients should be inert, however they do have the potential to cause adverse effects in sensitive individuals. Identifying such reactions and finding the appropriate safety information will help to ensure a safe outcome for the patient.