Why is cigarette smoking so addictive when its psychoactive effect is comparatively so subtle? The reason is that the cigarette is the Galil assault rifle of the nicotine delivery world: fast and reliable. Consider that while a heroin user injects a hit and feels a potent ephoric rush about fifteen seconds later, he is not going to inject again for many hours. The cigarette smoker, on the other hand, will typically take ten puffs from a single cigarette and will often smoke many cigarettes in the course of a day. Each puff will deliver nicotine to the pleasure circuit about fifteen seconds later, approximately the same delay as for intravenous heroin. So while a typical heroin addict may get two strong, rapidly delivered hits per day, the pack-a-day cigarette smoker will get two hundred weak, rapidly delivered hits per day. But why does the nearly instant delivery of a drug to the brain, as with smoking cigarettes or injecting heroin, carry a higher risk of addiction than slow delivery of the same drug, say, by chewing tobacco or eating opium?
One way to think about this is to consider that addiction is a form of learning. When someone uses a drug, associations are made between a particular act (injecting the drug or chewing the tobacco) and the pleasure that follows. Imagine that you have a dog that you're trying to train to come when called, using a tasty morsel of food as a reward. If you want to create a learned association, you'll call the dog, and when it comes you'll immediately give it the treat. Now imagine that instead of presenting the reward immediately (as with injected heroin), you wait thirty minutes and then offer the reward (as with ingested opium). In the latter case, the connection between the behavior (coming when called) and the reward is quite weak, and the association is less likely to be learned. The same dog-training anaolgy holds true for injected heroin (one big pleasure rush) and cigarette smoking (many tiny pleasure rushes). If you call the dog once a day, and then immediately reward its compliance with a ten-ounce steak, it will eventually learn to come when called. If you call the dog twenty times per day and immediately reward each correct behavior with a small chunk of meat, the dog will learn much more quickly. So when we smoke cigarettes, we are being very effective trainers of our inner dog, creating a strong association between puffing and pleasure.