This is the 2007 version. Click here for the 2017 chapter 03 table of contents.

Psychoactive Drugs

Drugs that affect states of consciousness are called psychoactive drugs. In this part of the chapter we will review the physical and psychological effects of many different substances. An extensive discussion of addiction is found later in Chapter 14 (Frontiers of Psychology).

All psychoactive drugs seem to have certain things in common.

What characteristics do all psychoactive drugs have in common?

1. They are able to cross the blood/brain barrier. In order to affect consciousness, a drug must penetrate the biological filter that prevents many other substances from reaching the brain.

2. They alter brain chemistry at the level of individual brain cells. Most drugs act at the level of neurotransmitters, for example, by blocking their re-uptake, interfering with their synthesis, or mimicking their effects. Some drugs alter the permeability characteristics of cell membranes or interfere with metabolic processes in neurons.

What is a dose/response curve?

3. Their effects depend on dosage. Researchers are able to vary the amount of the drug and observe resulting changes in behavior, from no change at all (at very low dosages) to toxicity or poisoning (at high levels). A dose/response curve is a graph relating dosage to some measure of behavior.

4. Their effects are altered by prior experience with the drug. Drugs can sensitize individuals to future exposures, leading to a larger reaction. Constant exposure to a drug leads eventually to the opposite phenomenon, tolerance, as the body re-establishes a new equilibrium in which presence of the drug is felt as normal. Few drugs produce the same effects in experienced users as they do in beginners.

What have researchers discovered about the failure rate for overcoming addictions?

5. Their effects depend on expectancy In all psychoactive drugs yet studied, the response to the drug depends partly on what people expect to happen. Individuals who think they are receiving alcohol will act drunk, even if they receive drinks without alcohol. Similar things happen with marijuana and other drugs.

6. They can be addictive. If a psychoactive drug produces pleasurable effects, some people will chose to repeat the experience. As they repeat the experience, their bodies adapt to the presence of the drug, sometimes producing uncomfortable or dangerous withdrawal reactions when they try to quit. Psychologists studying addictive behavior point out the failure rate for overcoming all addictions is about the same, whether the addiction is alcohol, cigarettes, or heroin.

What is a dose/toxicity ratio? Which drugs are most dangerous by this criterion?

One way to operationally define the danger of a drug is to calculate a dose/toxicity ratio. This is a ratio formed by dividing a typical dose of a drug by the amount that will kill a person. Writing in the journal American Scientist, Gable (2006) provided a summary of dose/toxicity ratios of common recreational drugs. It is on the internet here:

http://www.americanscientist.org/template/AssetDetail/assetid/50773?&print=yes

Gable reports that GHB and heroin "have a lethal dose less than 10 times their typical effective dose" while cocaine, MDMA (ecstasy), and alcohol "all have a lethal dose 10 to 20 times the effective dose." He continues:

A less toxic group of substances, requiring 20 to 80 times the effective dose to cause death, include Rohypnol (flunitrazepam or "roofies") and mescaline (peyote cactus). The least physiologically toxic substances, those requiring 100 to 1,000 times the effective dose to cause death, include psilocybin mushrooms and marijuana, when ingested. I've found no published cases in the English language that document deaths from smoked marijuana, so the actual lethal dose is a mystery. My surmise is that smoking marijuana is more risky than eating it but still safer than getting drunk.

Alcohol thus ranks at the dangerous end of the toxicity spectrum. So despite the fact that about 75 percent of all adults in the United States enjoy an occasional drink, it must be remembered that alcohol is quite toxic. Indeed, if alcohol were a newly formulated beverage, its high toxicity and addiction potential would surely prevent it from being marketed as a food or drug. (Gable, 2006)

What is capture ratio? Which drugs are most dangerous by this measure?

Gable notes that another, completely different operational definition of dangerousness in a recreational drug is capture ratio: the number of people who try the drug divided by the number who become regular or habitual users. He notes that "heroin and methamphetamine are most dangerous by this measure," followed by cocaine, alcohol, and nicotine, followed by caffeine and marijuana. Hallucinogens are least dangerous by this measure, because people do not often become dependent upon them. Of course, capture ratio disregards every other consequence of taking a drug except for the creation of dependence, so its usefulness is limited, but it provides a second way to measure danger, in addition to the dose/toxicity ratio.


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