VIENNA, Austria - Drug trafficking has become a $400 billion-a-year business worldwide, equal to 8 percent of all trade, the United Nations said today in its first comprehensive report on the illicit industry.

The report by the Vienna-based U.N. International Drug Control Program estimated that illegal drugs are bigger business than all exports of automobiles and about equal to the worldwide trade in textiles.

Seizures of drugs have been rising for about a decade, and the United Nations estimated that police now intercept 30 percent of all cocaine and 10 percent to 15 percent of heroin shipments. But demand and profits are so high that police work has barely dented the business. Drug traffickers find the risks worth taking.

"Profits on a mere fraction of the drugs successfully trafficked can cover the costs of the lost cargo," the report said. Three-quarters of all drug shipments would have to be intercepted to seriously cut into the profitability of the business, it said.

The 300-page report was the first effort by the U.N. organization to detail the worldwide drug business.

The world body said it hopes its broad description of the illicit industry will help law enforcement agencies attack it more effectively.

Publication of the report comes at a time when Secretary-General Kofi Annan is pressing the United Nations to get more deeply involved in the fight against drugs and organized crime.

The U.N. report said the abuse of synthetic drugs, primarily stimulants, has increased most rapidly.

Marijuana is the most widely used drug, the report said, with about 140 million users, or about 2.5 percent of the world population. However, it estimated the number of users of synthetic stimulants at 30 million - more than use heroin and probably more than use cocaine.

It reported a high level of use of methamphetamine in North America, the Far East and some Southeast Asian countries.

Use of marijuana and cocaine among U.S. eighth-grade students doubled between 1991-94, it said without providing details, and the average first use of marijuana was at 13 years old.

The estimated $400 billion annual revenue - 8 percent of total global exports of $4.95 trillion in 1995 - was generated by an industry encompassing poor farmers in Asia and South America, laboratories, an army of recruits to run the drugs, and a hierarchy that reaps the profits.

International organized-crime groups have plowed profits from other illegal activities such as smuggling cigarettes and jewels into the drug business. As the world financial network has expanded, money laundering has become more professional and more global.

The report estimated profit margins for methamphetamine at 240 percent; crack cocaine at 300 percent; and heroin at 100 percent. The average price of a kilogram of raw opium in Pakistan is about $90 but it sells for $290,000 in the United States.