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INTRODUCTION

Pharmacokinetics is the study of how the body absorbs, distributes, metabolizes, and eventually eliminates pharmacological compounds. In other words, what does the body do with the drug? This area includes the manner in which the drug is administered.

An introduction to pharmacokinetic principles will help you understand why specific drugs are administered in certain ways. Why, for example, can some drugs be administered orally while others need to be administered by injection, inhalation, or other non-oral routes? Likewise, drugs must reach a specific organ or “target” tissue to exert therapeutic effects, and various pharmacokinetic variables must be taken into account to maximize the drug’s ability to reach these tissues. Finally, it is critical to know how the body metabolizes and eliminates a drug so that you can be aware of problems that might arise if drug metabolism is altered by illness, disease, or other factors. This chapter will begin by considering various routes of administration. Other pharmacokinetic issues, such as drug absorption, distribution, and storage, will then be addressed. Drug metabolism and elimination will be covered in the next chapter.

ROUTES OF ADMINISTRATION

In general, drugs can be administered via two primary routes: the alimentary canal (enteral administration) or the nonalimentary routes (parenteral administration). Each route has several variations, and each offers distinct advantages and disadvantages. The key features of various routes are discussed here (see Table 2-1). For a more detailed description of the specific methodology involved in drug administration, the references at the end of this chapter include several excellent resources on this topic.1-3

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Table 2-1 Routes of Drug Administration

Route

Advantages

Disadvantages

Examples

Enteral

Oral

Easy, safe, convenient

Limited or erratic absorption of some drugs; chance of first-pass inactivation in liver

Analgesics; sedative-hypnotics; many others

Sublingual

Rapid onset; not subject to first-pass inactivation

Drug must be easily absorbed from oral mucosa

Nitroglycerin

Rectal

Alternative to oral route; local effect on rectal tissues

Poor or incomplete absorption; chance of rectal irritation

Laxatives; suppository forms of other drugs

Parenteral

Inhalation

Rapid onset; direct application for respiratory disorders; large surface area for systemic absorption

Chance of tissue irritation; patient compliance sometimes a problem

General anesthetics; antiasthmatic agents

Injection

Provides more direct administration to target tissues; rapid onset

Chance of infection if sterility is not maintained

Insulin; antibiotics; anticancer drugs; narcotic analgesics

Topical

Local effects on surface of skin

Only effective in treating outer layers of skin

Antibiotic ointments; creams used to treat minor skin irritation and injury

Transdermal

Introduces drug into body without breaking the skin; can provide steady, prolonged delivery via medicated patch

Drug must be able to pass through dermal layers intact

Nitroglycerin; motion sickness medications; drugs used with phonophoresis and iontophoresis

Enteral

Oral

The most common method of enteral ...

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