What are routes of administration for drugs of abuse?

Background

For drugs to affect a person’s psychological experience, they must reach the brain. This statement is rather obvious, but what may be less obvious is how the path a drug takes to the brain can affect the experience and consequences associated with taking the drug. All things being equal, the faster an addictive substance is delivered to the brain, the more that drug tends to be abused and addictive. One variable that plays a large role in determining how quickly a drug is delivered to the brain is the route of administration, or the method used to get the drug into the body.

Oral Route

Perhaps the most common route of administration is the mouth. The introduction of drugs into the body by swallowing them is referred to as oral administration. Pills are administered this way, as are liquids such as alcohol.

In addition, drugs that are consumed in whole-foods forms, for example, marijuana baked into brownies or hallucinogenic mushrooms brewed into tea, also are administered orally. When orally administered, drugs will be partially absorbed in the stomach and intestine, where the active ingredients are absorbed into the bloodstream and then carried to the brain.

The oral route of administration is a relatively slow way to deliver drugs to the brain. Because the active ingredients are absorbed through digestive processes, the speed of onset can be influenced by a variety of stomach and intestinal variables. A prominent example of this process is alcohol consumption, in which approximately 20 percent of the alcohol is absorbed in the stomach and the rest in the upper intestine. The speed with which this occurs is affected by how fast the stomach empties; this, in turn, is related to how much food is already in the stomach. This factor explains why people are more rapidly influenced by the effects of alcohol when they drink on an empty stomach.

In addition, men have more of a particular gastric enzyme that metabolizes a portion of the consumed alcohol before it is absorbed into the bloodstream. This partly explains why men will be somewhat less influenced by the effects of alcohol than will women, even when the number of drinks and body size are held constant.

Injection

A second route of administration is intravenous (IV), intramuscular, and subcutaneous. In an IV administration, the drug is injected with a syringe and needle, often directly into the bloodstream through a vein. Drugs also can be injected directly into muscle (an intramuscular injection) or just under the skin (a subcutaneous injection). However, most drugs of abuse that are taken through self-injection are delivered intravenously.

IV injection is relatively dangerous because it is fast and irreversible. Once the drug has been injected, it cannot be retrieved; the injection cannot be reversed. In addition, IV injection bypasses virtually all barriers to absorption. The drug that is injected will be completely and rapidly available. In some cases the rapid availability of the drug can itself become a problem. The dose of a drug that is administered orally will gradually become available to the brain as the dose dissolves and is absorbed. The same dose delivered intravenously will be available to the brain all at once. Serious problems can occur if the user has an allergic reaction, miscalculates the dosage, or has an unanticipated side effect.

Finally, it can be difficult for IV drug users to obtain safe and clean needles. Shared or reused needles can spread infection and blood-borne diseases, including hepatitis virus infection and infection with the human immunodeficiency virus.

Inhalation

among drug users is the lungs, in which the substance of abuse is inhaled. The inhalation route includes smoking, in which the drug of abuse is heated or burned to suspend its particles in smoke or vapor.

Inhalation is a popular way to administer tobacco, marijuana, and smoked versions of cocaine, methamphetamine, and heroin. Also, chemicals that are found in many household products, including some paints, cleaners, and glues, also can be administered as substances of abuse if their fumes are inhaled. The lungs absorb the inhaled chemicals rapidly and efficiently and send them directly to the heart and brain. As a consequence, drugs that are inhaled will have an extremely fast onset, sometimes even faster than IV injection.

Inhalation has the potential to damage the lungs themselves. In many cases the lung damage is not directly caused by the drug itself, but by other drugs and toxins that are delivered to the lungs at the same time. An additional problem associated with inhalation is that the amount of oxygen absorbed and made available to cells of the body is often reduced. Reducing the amount of oxygen available to the brain leads to a dangerous condition called anoxia.

Anoxia is a common and especially devastating problem with the abuse of chemicals in household cleaners, glues, and paints. In this case, the anoxic effects produced by the chemicals themselves are compounded by efforts that inhalant abusers often take to concentrate the gasses they are inhaling, such as covering the mouth with chemical-soaked rags or plastic bags.

Other Modes of Administration

Drugs also can be absorbed through nasal or oral mucous membranes (mucous administration). This route of administration is common with tobacco chew and snuff, in which the nicotine is absorbed through tissue in the mouth and nose, respectively. Mucous administration also is the method used to absorb cocaine or powdered forms of other drugs, which are snorted (sniffed into the nose).

Drugs also can be absorbed directly through the skin (transdermal administration). The most familiar example of transdermal administration of drugs is the nicotine patch, but this strategy is gaining popularity for the delivery of other drugs, including pain medications.

Bibliography

Julien, Robert M., Claire D. Advokat, and Joseph E. Comaty. A Primer of Drug Action. 12th ed. New York: Worth, 2011. An excellent, thorough text covering uses and effects of both psychiatric substances and drugs of abuse.

McCabe, Sean E., Carol J. Boyd, and Christian J. Teter. “Motives, Diversion, and Routes Of Administration Associated with Nonmedical Use of Prescription Opioids.” Addictive Behaviors 32 (2007): 562–75. Print. Reports the results of a large web-based survey of undergraduate students investigating the ways that students abuse opioid medications for nonmedical purposes and their motivations for doing so.

Samaha, Anne-Noel, and Terry E. Robinson. “Why Does the Rapid Delivery of Drugs to the Brain Promote Addiction?” Trends in Pharmacological Sciences 26 (2005): 82–87. Print. Proposes a theory of brain function and summarizes alternative theories for why increasing the speed of delivery of drugs to the brain increases their addictive potential.