“Chasing the tail of the dragon” is a drug culture expression meaning that the initial “high” of drug use cannot be re-experienced by using more drugs.
It is a futile quest because that initial high wasn't in the drug, but in the brain's chemical response.
Long after pleasure is replaced by discomfort, the addicted person will continue using in a vain attempt to have a sensation that recedes further and further from them with each attempt to attain it.
The drug user who is not addicted knows that “chasing the tail of the dragon” is a waste of time, money, emotion and, of course, drugs. When the intoxication approaches a certain level, they “know when to say when.” For drinkers, this is called “drinking responsibly.” When non-addicted drug users find that a drug no longer “works,” they simply stop using it.
Persons with addiction, however, compulsively use long after the the thrill is gone, even when using causes blatantly unpleasant physical,.emotional and social consequences. This isn't because they “don't know any better,” or that they are intentionally hurting themselves. The reason, simply put, is that the “knowing better” part of the brain – the area of executive function and judgment – is temporarily out of order. They are not using drugs against their better judgment. They are continuing to use because their better judgment is not functioning.