29 Following the administration of a medicine, the drug has to reach its site of action or receptor in order to produce an effect. How this is achieved is often a complex process affected by many factors. The first stage will be the release of the drug from the dosage form, to be followed by absorption into the body (unless it is for a surface effect at the site of administration). There is then a distribution process, usually in the blood, which will take the drug to the site of action. As soon as it is in the body, metabolic processes, especially in the liver, will start to change the drug and the elimination process will also commence. A detailed discussion of these processes is outside the scope of this book, although it does have a significant impact on the choice of both the route of administration and the actual dosage form. There is a growing awareness that the correct choices can have an important impact on therapeutic outcomes for the patient. This chapter will review the various routes of administration used for drug delivery and discuss some of their advantages and disadvantages. Brief details of a variety of dosage forms are also given. Figure 29.1 illustrates the principal routes of administration. However, there are disadvantages which should be borne in mind: Two sites are used for absorption from the buccal cavity: It is important that patients are made aware of the difference between the two sites and they should be given full instructions on how to administer their tablets, to ensure maximum benefit. (For details of suitable patient instructions, see Ch. 39.) The advantages of the buccal route: For administration by this route, drugs are formulated as liquids, solid dosage forms and semi-solids (see Ch. 37). The chosen preparation is inserted into the rectum from where the drug is released to give a local effect or it may be absorbed to give a systemic effect. The amount of fluid present in the rectum is small, estimated at approximately 3 mL of mucus. This affects the rate of dissolution of the drug released from the suppository. However, there is also muscular movement which spreads the drug over a large area and promotes absorption. The advantages and disadvantages of this route of administration are as follows. Drugs are administered usually by inhalation through the nose or mouth to produce either local or systemic effects. This route is used predominantly for local administration to treat respiratory conditions such as asthma. For this, drugs are delivered directly to the site of action, i.e. the lungs. A variety of dosage forms are used, from simple inhalations consisting of volatile ingredients such as menthol to sophisticated inhaler devices (see Ch. 43). A major benefit of the inhaled route is that the drug dose required to produce the desired effect is much smaller than for the oral route, with a consequent reduction in side-effects. Because of the high blood flow to the lungs and their large surface area, drug absorption by this route is extremely rapid and can be used to give systemic action.
Routes of administration and dosage forms
Introduction
Routes of administration
The oral route
From a patient’s point of view it is the simplest route
Self-administration of drugs can be carried out
The buccal routes
For sublingual absorption, the area under the tongue is used. This gives a very fast onset of action of the drug but duration is usually short
The rectal route
The inhalation route
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