It sounds at first as if the White House drug policy director is making a joke.

"You can divide the country into two groups," he says. "The group that does not use drugs, and the group that does. We've had great success with the first group."

But Donald Ian MacDonald intends no irony. His point is that the effort to keep young people from starting with illegal drugs has paid off. But the effort to get drug abusers to quit has had only marginal success -- among cocaine users, practically no success at all.

"Americans have become very intolerant of drug use, just as they are becoming more intolerant of cigarette smoking," he said in an interview, "and the numbers reflect it. There has been an almost steady decrease in the use of most drugs among high school seniors over the last seven years. The daily use of marijuana is down from 7 percent to just 4 percent. That's almost 200,000 fewer high school kids using marijuana."

But the picture for cocaine is far less optimistic. The number of youngsters answering yes to the question, "Have you used cocaine within the last month?" grew from 4.3 million in 1982 to 5.8 million in 1985. The "yes" responses held steady at about 12 million for the question, "Have you used cocaine in the last year?"

MacDonald contends that those numbers, from the National Household Survey on Drug Abuse, reflect an increase in the intensity of cocaine abuse -- not more users but more frequent use among users.

"What we are looking at," he said, "are people who thought they could use cocaine occasionally without becoming addicted to it, only to learn that they couldn't. The emergence of crack {cocaine in its crystalized, smokable form}, which is almost instantly addictive, has greatly accelerated the process of both addiction and toxicity.

"As we are able to increase the level of knowledge about drugs, we are able to change attitudes and behavior. Young people now understand that these drugs are addictive, and their use is declining."

Thus the decreases in the use of LSD and PCP, as well as marijuana. The increase in the frequency of cocaine abuse and the steady rate of heroin abuse, MacDonald believes, are evidence of addicted users rather than new users.

Does he claim credit for the decrease in drug use?

"As I said, we're doing well in the nonuser population," he said. "Mrs. Reagan's efforts have been of major importance, and our educational programs have been a factor in changing the public attitude toward drug use.

"That being said, I have to tell you that all these prevention campaigns are unlikely to change the behavior of the millions of Americans who use illicit drugs every month. Who are they? Mostly they are in the 20-to-40 age group, the population that did not have the benefit of the knowledge we have now."

These were the teen-agers and young adults of the 1960s who disregarded the warnings against drug use, believing that they were mostly exaggerations and scare stories designed to produce conformity. But if ignorance tempted them into addiction, knowledge won't necessarily cure them, MacDonald believes.

"The next step has to be against the users. As with any addiction, the first step of the cure is to assume responsibility for your own behavior. You can talk about stress or frustration or insecurity, all of which may be true, but individuals have to acknowledge that they chose to use drugs and they can choose to stop."

The role of society, in MacDonald's view, is to stop offering excuses and expose people to the consequences of their bad choices. He cites the example of Missouri, which recently passed a law that postpones the issuance of drivers' licenses for teen-agers convicted of drug abuse. Republican presidential candidate Pierre S. du Pont has proposed annual drug testing for all teen-agers on a random basis, with a two-year delay in driver-license eligibility for those found to be using drugs.

"What we are talking about isn't so much introducing punitive measures as changing our present attitude, which is enabling," MacDonald said. "Our present attitude, which allows drug abusers to blame someone else -- 'society' -- for behavior that is destructive to themselves or others, enables them to continue that behavior. It's like having your teen-ager smash up the family car several times and you only say, 'Be more careful.' At some point, we have to think in terms of consequences.

"People talk about the failure of Prohibition. In my view, the mistake we made with Prohibition was that virtually all of the effort was directed against bootleggers. Use or possession of alcohol was rarely punished. Contrast that with Japan, which faced a major amphetamine epidemic in the 1950s. Japan's approach was user-based. They cracked down and arrested 54,000 amphetamine abusers, and in two years the epidemic was over."