Pharmacokinetics

Absorption

Absorption can be defined as the movement of a drug into the blood stream, which is dramatically affected by the route of drug administration (Table 2.2). By definition, intravenous administration goes directly into the veins, giving instantaneous absorption. Each route of administration has advantages and disadvantages. The intravenous route is highly titratable, hence the amount of drug administered over time can be controlled very carefully. One disadvantage, however, is that there is no turning back. Once absorbed intravenously, there is no easy way to remove the drug from the bloodstream before it enters the brain. The oral route, by contrast, is highly variable and generally less preferred by those who use addictive drugs, but it is generally the safest route of administration. The time course of the effects of a drug is also highly affected by the route of administration (Figure 2.4). Particularly relevant to drugs of abuse is the general principle that faster absorption is associated with a higher likelihood of abuse. For example, amphetamines have high addiction potential when injected intravenously, but Adderall (a mixture of amphetamine isomers/salts) appears to have low addiction liability when used orally and appropriately for the treatment of attention-deficit/hyperactivity disorder.

TABLE 2.2

Common Routes of Drug Administration

Route

Advantages

Disadvantages

Intravenous

Valuable for emergency use
Titratable
Suitable for large volumes
Suitable for irritating substances

No turning back
Increased risk of adverse effects
Must inject slowly

Subcutaneous

Suitable for some insoluble suspensions
Suitable for implantation of solid pellets

Not suitable for large volumes
Possible pain
Possible tissue damage

Intramuscular

Suitable for moderate volumes
Suitable for some irritating substances

Contraindicated when concomitant with anticoagulants
Possible pain
Possible tissue damage

Oral

Reversible
Generally safe
Convenient
Economical

Requires patient cooperation
Erratic absorption
Possible incomplete absorption
Possible instability

Figure 2.4 Time course of blood levels of drug by different routes of administration.

The basis of the differences in absorption via different routes of administration depends on several factors. One obvious factor, however, is the number of physical membranes within the body that the drug needs to cross before it can be absorbed into the bloodstream (Figure 2.5). Drugs that enter the body through the gastrointestinal track, skin, or lungs must first cross an epithelial barrier and then the endothelial cells of capillary walls. Drugs that are administered subcutaneously or intramuscularly bypass the epithelial barrier but must also cross the endothelial cells of capillary walls. Epithelial cells line the cavities and structures of the body. Endothelial cells line the interior surface of blood vessels.

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