Imagine you are taking a slug of whiskey. A puff of a cigarette. A toke of marijuana. A snort of cocaine. A shot of heroin. Put aside whether these drugs are legal or illegal. Concentrate, for now, on the chemistry. The moment you take that slug, that puff, that toke, that snort, that shot, trillions of potent molecules surge through your bloodstream and into your brain. Once there, they set off a cascade of chemical and electrical events, a kind of neurological chain reaction that ricochets around the skull and rearranges the interior reality of the mind. Given the complexity of these events and the inner workings of the mind in general it’s not surprising that scientists have struggled mightily to make sense of the mechanisms of addiction. Why do certain substances have the power to make us feel so good ? Why do some people fall so easily into the thrall of alcohol, cocaine, nicotine and other addictive substances, while others can, literally, take them or leave them? The answer, many scientists are convinced, may be simpler than anyone has dared imagine. What ties all these mood-altering drugs together, they say, is a remarkable ability to elevate levels of a common substance in the brain called dopamine. In fact, so overwhelming has evidence of the link between dopamine and drugs of abuse become that the distinction between substances that are addictive and those that are merely habit-forming has very nearly been swept away. The Liggett Group, smallest of the U.S.’s Big Five cigarette makers, broke ranks in March and conceded not only that tobacco is addictive but also that the company has known it all along. While RJR Nabisco and the others continue to battle in the courts insisting that smokers are not hooked, just exercising free choice their denials ring increasingly hollow in the face of the growing weight of evidence. Over the past year, several scientific groups have made the case that in dopamine-rich areas of the brain, nicotine behaves remarkably like cocaine. And late last week a federal judge ruled for the first time that the Food and Drug Administration has the right to regulate tobacco as a drug and cigarettes as drug-delivery devices. Now, a team of researchers led by psychiatrist Dr. Nora Volkow of the Brookhaven National Laboratory in New York has published the strongest evidence to date that the surge of dopamine in addicts’ brains is what triggers a cocaine high. In last week’s edition of the journal Nature they described how powerful brain-imaging technology can be used to track the rise of dopamine and link it to feelings of euphoria. Like serotonin , dopamine is a neurotransmitter a molecule that ferries messages from one neuron within the brain to another. Serotonin is associated with feelings of sadness and well-being, dopamine with pleasure and elation. Dopamine can be elevated by a hug, a kiss, a word of praise or a winning poker hand as well as by the potent pleasures that come from drugs. The idea that a single chemical could be associated with everything from snorting cocaine and smoking tobacco to getting good grades and enjoying sex has electrified scientists and changed the way they look at a wide range of dependencies, chemical and otherwise. Dopamine, they now believe, is not just a chemical that transmits pleasure signals but may, in fact, be the master molecule of addiction. This is not to say dopamine is the only chemical involved or that the deranged thought processes that mark chronic drug abuse are due to dopamine alone. The brain is subtler than that. Drugs modulate the activity of a variety of brain chemicals, each of which intersects with many others. “Drugs are like sledgehammers,” observes Dr. Eric Nestler of the Yale University School of Medicine. “They profoundly alter many pathways.” See TIME’s Pictures of the week.