Why The Route Of Administration Matters

Because so much of the heart's output of blood goes to the brain, any route of administration that deposits a large amount of drug into the blood quickly will promptly get a large portion of that drug to the brain. In contrast, a route that gets the drug into the blood more slowly decreases the rate at which it gets to the brain and, because of the way the body eliminates drugs, will probably also reduce the amount of drug that finally does reach the brain. Why this is so will be described in more detail below. Most drug abusers and addicts have learned the two most effective ways to deliver lots of drug into the brain quickly: (1) inject the drug directly into the bloodstream or (2) smoke it.

Intravenous Injection: One Fast Track

Doctors and nurses routinely inject medicine into veins because this is an extremely rapid, reliable, and efficient way of getting medicines into the blood and distributed throughout the body. Drug users inject drugs for the same reasons. When a heroin addict injects a dose into a vein, a bolus (concentrated mass) of the drug enters the blood in a couple of seconds. Within a few heartbeats, the heroin passes into the right side of the heart, where all the blood returning from the brain and body goes first. A few more heartbeats send the heroin through the lungs, where the blood goes to replenish its oxygen supply, and then back to the heart, this time to the left side. From there, it takes only a couple more beats to get the heroin to the user's brain and body.

One major reason heroin addicts shoot up intravenously is to obtain a "rush," an orgasm-like feeling that seems to emanate from the pit of the stomach. Taking the same dose by mouth or through the skin or nose just won't do it. The presence of a rush after intravenous administration—not by other routes—shows how important the rate at which a drug gets to the brain can be in producing certain effects.

Addicts who overdose and die are sometimes found with the needle still in their veins. Actually, they don't die right away. What happens is that the effect of the injected drug is so fast and potent that the user loses consciousness before completing the injection and removing the needle. The user dies soon after because the heroin has shut down the brain's messages that tell the lung muscles to contract and breathe.

Smoking Drugs: The Other Fast Track

Drug abusers have also found that smoking can put a large amount of a drug into the bloodstream quickly. But, only drugs that can literally be vaporized (i.e., turned into a gas) can be inhaled with good results. Burning the drug itself inactivates the drug. Powder cocaine burns at a lower temperature than the one required to vaporize it; so smoking powdered cocaine produces little or no effect. The drug is burned off before it can be vaporized. Crack cocaine, by contrast, vaporizes at a lower temperature than it burns, so it is ideal for smoking. The difference between the two forms of cocaine is a minor chemical modification that has no effect on the pharmacological actions, but does change the temperature at which the compound will vaporize, turning from a solid into a gas. Crack cocaine, the tetrahydrocannabinol (THC) in marijuana, and the nicotine in tobacco all are examples of drugs that vaporize before they burn and can therefore be smoked effectively.

People can also smoke methamphetamine and heroin. While a portion of these drugs is being vaporized, however, some burns up as well, so the user needs a lot of drug to produce the desired effect. Drug dealers usually "cut" (dilute) drugs like heroin, cocaine, and methamphetamine with inactive additives (such as baking soda or fine sugar) to increase the amount the user seems to be buying. As these drugs have become more prevalent over the last couple of decades, dealers have been using smaller amounts of additives.

In the 1970s, a "bag" of heroin (the common unit of sale) was only about 4% heroin. By the late 1990s, heroin potency had risen dramatically to 40%. To contemporary users, it seems as if the drug they buy is more pure (or more potent). The active drug is no different, there is just more of it in each bag. It is this increase in "purity" that has made the smoking of heroin and methamphetamine cost-effective for users.

Smoking Tobacco: All the Advantages of Smoking Revealed

Drug smokers take advantage of the efficient system evolution has given us for absorbing gases through the lungs. In the lungs, the body can absorb inhaled gases from the air into the bloodstream by allowing them to pass through only two layers of cells. One layer lines the lung itself. The other layer forms the walls of the capillaries whose job is to bring blood to the lungs to pick up oxygen and dispose of carbon dioxide. The lung itself consists of tiny balloon-like air sacks called alveoli. Gases pass through the one-cell-thick walls of the alveoli and into the capillaries, which are directly adjacent to them. Many hundreds of millions of these alveoli (they are very small) are in the chest. This is why, as noted earlier, our lungs have a surface for absorption about as large as a tennis court.

During normal breathing, we fill only 25% of our lungs' alveoli with new air. Although she is not particularly aware of this, when Sybil smokes (see Chapter 2), she tries to take advantage of more of the lung's absorptive surface area. She takes deep breaths, expands her lungs, and draws the smoke down into many more of the alveoli, holding her breath to allow time for her blood to absorb the nicotine from all the smoke. She often inhales several times in rapid succession, filling her lungs with a lot of nicotine in a relatively short amount of time so she can increase her nicotine blood levels quickly. From her lungs, it is only a few heartbeats back to the left side of her heart before her blood distributes nicotine to her brain and throughout her body.

Titration of Dose

In addition to getting a lot of drug to the brain quickly, drug users can use smoking a second way. They can titrate (adjust) the amount of drug in the blood and keep it relatively constant over long periods of time. Cigarette smokers especially find this advantageous. For example, once Sybil has taken her first couple of nicotine hits in the morning, she usually takes fewer puffs, fills her lungs with less smoke, and slows down her smoking rate. If she smokes slowly instead of rapidly, she can perceive the effect of the drug and stop smoking when she reaches the desired level of effects she seeks. If that level falls too low, she lights up again and takes in what she needs to titrate her blood nicotine to the higher level she prefers.

As you have probably guessed, all cigarette smokers typically adjust the amount of nicotine in their blood by controlling how frequently they smoke, how quickly they smoke each cigarette, and even how much of each cigarette they consume. It doesn't take too long for even a novice smoker to learn to detect the first subtle signs of nicotine withdrawal and interpret those signs as a signal to light up. Most experienced smokers have learned to titrate their nicotine dose to meet their needs. This is doubly important because nicotine is a toxic substance, and too much can produce unpleasant side effects, such as dizziness and nausea. Most novice smokers encounter these side effects when they first start smoking, but become tolerant to them quickly. After just a little practice, they are able to adjust their nicotine dose to obtain the desired effects while avoiding the unpleasant ones.

Marijuana users can regulate their input in a similar fashion. However, the marijuana smoker must be patient because the effects of marijuana are not felt immediately, and it is easy to become very intoxicated by smoking too much too quickly. The ability to titrate doses has been used as an argument for making marijuana available for medical uses. Now, doctors can prescribe an orally administered drug containing a synthetic version of THC (dronabinol, trade name Marinol), the major active ingredient in marijuana, for the nausea that results from cancer chemotherapy and to treat the wasting syndrome of AIDS by increasing appetite. However, marijuana advocates argue that the oral dose is too high, and patients could benefit more from smokable marijuana, because they could titrate their own dose. We will address this issue in more detail in Chapter 11.

Taking Drugs by Mouth

People cannot inject or smoke all drugs. Other routes of administration, though less effective in rapidly creating high blood levels of a drug, nonethe less effectively get drugs into the body. For example, people can take most drugs by mouth. Indeed, at the end of the 19th century when opiates and cocaine were legal, a number of medicines available over the counter or by mail contained opiates and cocaine. One wine even contained cannabis, and some popular drinks contained cocaine (an original ingredient in Coca-Cola).

Physicians usually prefer to give medications in oral form, because this makes dosing easy. The patient can take the drug by herself and will get a specific amount of the drug each time she takes it. Doctors can thereby regulate the amount of drug in the patient's body over long periods of time.

In terms of the number of users, the most popular psychoactive drug taken by mouth is alcohol. A wide variety of tasty (and not so tasty) drinks contain alcohol, and drinking alcohol is part of many complex social rituals. Wine plays an important role in the rituals of several religions, including Catholicism and Judaism. Common social events, like going out to dinner, may be surrounded by rituals that include drinking sweet, tasty cocktails, fine wines, and aged whiskeys. If all drinking were confined to these kinds of situations, alcohol would not be the problem that it is today. Unfortunately, tasty alcoholic concoctions allow us to ingest large quantities of a naturally toxic substance, which by itself tastes really awful. And all around the world, people drink these large quantities and get into trouble because of them.

So, it is evident that taking drugs by mouth is an effective way of getting them to the brain. The journey from the mouth to the brain is more complicated, however, than the trip that begins inside a vein or the lungs.

How Drugs Taken by Mouth Reach the Brain

Drugs people take by mouth are absorbed through the stomach, the intestines, or both. If you have any experience with alcohol or any common pain reliever for a headache, toothache, or similar pain, you know it usually takes at least 10 minutes before you even begin to feel the effects, and it can take much longer if you have food in your stomach and intestines. So, drugs taken by mouth get into the bloodstream relatively slowly. Not only that, once drugs enter the stomach, they are exposed to all the digestive chemicals and enzymes waiting to digest food. Indeed, our stomachs contain alcohol dehydrogenase, an enzyme that breaks down and inactivates alcohol. Men's stomachs contain more of this enzyme than women's, which is one reason most women are more sensitive to the effects of alcohol than most men. Destruction of drugs by the stomach and intestines means that for an effective dose to reach the brain, more must be taken than if the same drug were inhaled or injected intravenously.

Drugs such as alcohol and aspirin must pass through the walls of the stomach and intestines and into capillaries that will deposit them into the veins. The intestines in particular are specialized to facilitate this process, but it still takes much more time for an ingested drug to reach the bloodstream than for a vaporized drug to pass from the lung to the blood. Even when the drug has reached the blood, it has one more hurdle to cross before it can reach the heart and then the brain. That hurdle is the liver.

All the blood that leaves the stomach and intestines passes through the liver before returning to the heart. This passage is important because the liver is loaded with enzymes that can inactivate (metabolize) all kinds of chemicals (including drugs) that might arise normally from metabolism or that we might ingest, either by accident or on purpose. In effect, the liver is a big filter for most chemicals that pass into the body from the gastrointestinal tract. So, after some of the ingested drug is lost in the stomach and intestines, even more is lost once the liver does its work. These two factors are the major reasons why medicines taken by mouth must be given in larger doses than those that are injected.

Although any substance that gets into the blood eventually passes through the liver, blood from the gastrointestinal system goes to the liver to be cleansed before it goes anywhere else. This cleansing of the blood on its first pass through the liver has a profound effect on reducing drug concentration in the blood. For example, the standard dose of morphine given in a hospital for severe pain is 10 mg when injected, or 60 mg when given by mouth. This sixfold difference reflects the effects of the gastrointestinal tract and liver on reducing the amount of drug that is delivered into the body.

But there is another difference between injected drugs and drugs taken by mouth. Because of the increased time required for absorption, the onset of the effects of a drug are slower when people take it by mouth than when they inject or smoke it. As a result, the peak effect of the drug may not be as striking. For example, addicts inject heroin intravenously to produce a rush. Heroin taken by mouth produces at best a small rush, even if enough is taken to produce blood levels equal to those attained through intravenous injection. Simply changing the route of administration from injection to ingestion eliminates one of the key effects sought by heroin addicts. We should not be surprised, then, that heroin addicts "mainline" (inject). Users of all kinds of drugs typically select routes of administration that maximize the drug effects they are seeking.

How Drugs Taken Through Mucous Membranes Reach the Brain

Mucous membranes, like the insides of the mouth and nose, are the linings of the various body cavities designed to allow things (such as food, air, or body wastes) to enter or leave the body. Unlike skin, they are kept moist, and certain kinds of chemicals can dissolve in this moisture and thereby gain entrance to the body. People can take a few drugs, like LSD (lysergic acid diethylamide), cocaine, and even heroin, into their bodies by absorbing them through the mucous membranes that line the mouth, nose, throat, vagina, and rectum. The moist surfaces that line the body's orifices allow passage of many kinds of chemicals and are near the blood supply.

Absorption through mucous membranes is not a particularly effective process, but it does work. People commonly take LSD by placing a "tab" under the tongue and letting the drug be absorbed there. (LSD is so potent that very small amounts, measured in micrograms, are effective. A dose of LSD is a tiny drop that is typically placed onto a small square of blotter paper. The drug user places the paper into his mouth, and his saliva absorbs the drug. The piece of blotter paper is the tab. Some of the other nicknames for LSD, such as "window pane" or "sunshine," refer to graphic designs made on the blotter paper.) Some medicines, notably the nitroglycerin that people take to treat angina, are also administered by this route. LSD abusers don't mind the slow onset of effects, because LSD does not produce a heroin-like rush by any route of administration.

In addition, people can snort cocaine or heroin into the nose, where it is absorbed from the upper respiratory tract. This route of administration is a little faster and more efficient than taking the drug by mouth and absorbing it through the stomach and intestines. Moreover, both cocaine and heroin leave a bitter taste that can be avoided by snorting them up the nose. If nothing else, snorting protects the drug from being digested.

Because heroin is now so much more pure than it used to be, snorting it has become an effective and popular route of administration.

Continue reading here: The Drug Abuse Control Laws

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