AT A TIME when heroin use and addiction in the United States has decreased significantly, Western Europe, in the words of State Department and White House drug officials, is "swimming in smack." These countries lack a comprehensive drug-use reporting system. But the available evidence indicates a sharp rise in use and addiction, although the number of users is small in absolute terms and smaller still when compared with the 500,000 heroin addicts in the United States. The rapid spread has European governments worried and has erased much of the indifference they showed when heroin was largely an American problem.

The Italian Parliament in 1975 passed a comprehensive drug law that decriminalized all personal possession of every drug and mandated government-sponsored services for drug users. (Unfortunately, the government's instability has prevented any attempt to carry out the law on a large scale.) In France a special commission on drug use will soon make public its report. And in West Berlin, which experienced a 50 per cent increase in heroin-overdose deaths last year, the city government is about to set up a $4-million antiheroin program.

The heroin now flooding Western Europe is coming largely from opium-growing areas in Pakistan and Afghanistan along a supply route that makes use of produce-carrying trucks from Turkey or the frequent charter flights that bring Turkish workers and their families to West Germany. European officials must shut down this network, just as several years ago American and French officials closed the network that was supplying the United States with heroin made from Turkish opium. But American drug officials, out of their own experience, can tell their European counterparts that once the "Turkish connection" is shut off, they can expect an influx from some other part of the world: Southeast Asia, Mexico, South America. For example, American and Mexican drug officials are now trying to close down the "Mexican connection," which supplies nearly 70 per cent of the heroin that reaches the United States. This network sprang up after the American-Turkish supply route was closed.

Can the international traffic in drugs be eradicated? Probably not. But it can be diminished. The traffic in illicit drugs is international in scope. Attempts to restrict it must be, too. The United States, because its huge drug problem forced it to act, has shown what should be done: Expand drug-enforcement agencies and drug-treatment programs. Seek the cooperation of the "supply" countries in destroying the drugs at the source. And support the various U.N. committees and projects concerned with the illicit drug trade. U.S. officials say Western European governments are taking a more active interest in -- and increasing their contributions to -- the U.N. groups. That's welcome news. But much more remains to be done.