While numbers of U.S. heroin and cocaine users may have leveled off or decreased, the President's Commission on Organized Crime has found a dangerous increase in the amount of drugs used, deaths from overdoses, simultaneous use of dangerous drugs and their usage by middle-class Americans.

The commission, in a major report this week, said that while the number of heroin users has remained almost constant since 1979 at just under 500,000, down from 740,000 two years earlier, the quantity consumed has increased and heroin-related deaths jumped by nearly one-third from 1983 to 1984.

In contradiction to the common belief that heroin use leads to immediate addiction, the commission found a new group of nonaddicted "chippers," who use heroin occasionally over a long period without becoming addicted.

The commission also reported a new phenomenon: middle-class heroin users, addicts and nonaddicts, who began to use the drug because of their dependency on cocaine.

"In order to counteract the effects of heavy cocaine use, many begin to use heroin intranasally and eventually become addicted to the drug," the report said.

The panel quoted the operator of a Los Angeles drug-abuse clinic as saying:

"As cocaine use increases, we're seeing more [heroin] addicts because heroin is the strongest neutralizer around for cocaine toxicity . . . . We're not talking about gang members and derelicts. I'm treating people who pay their union dues, go to the PTA, take their kids to Little League. We've even got a program for executives."

The commission report also disputed as myth that cocaine use is limited largely to wealthy Americans.

It said cocaine is used "by individuals of all socio-economic groups," producing $11 billion in illicit income annually. Surveys of high school seniors indicated that cocaine use increased from 4.9 percent in 1983 to 6.7 percent in 1985.

The commission found that the number of cocaine users also has leveled while consumption increased 12 percent from 1983 to 1984. According to the report, 25 million Americans have tried cocaine, while 5 million to 6 million people use it at least once a month. Of those regular users, about half are considered addicted, it said.

Meanwhile, the commission found that cocaine deaths increased by 77 percent from 1983 to 1984, while cocaine-related emergency-room visits increased 51 percent during that period. Over the last three years, it found, requests for treatment for cocaine abuse increased by 600 percent.

To back its claims on increased use of overlapping drugs, the commission noted that 59 percent of the cocaine-related deaths in 1984 involved other drugs, especially alcohol, heroin and PCP. It also cited an estimate that the use of speedballs," dangerous injections of heroin and cocaine, increased 37 percent from 1983 to 1984.

The report said that between 20 and 25 percent of Americans have tried marijuana and that 20 million use the drug at least once a month. Although it found marijuana use "firmly entrenched in American society," it urged the 11 states that have decriminalized personal marijuana use to consider repealing those laws.

Synthetic drugs, including stimulants, depressants, hallucinogens and synthetic narcotics were used by 6 million people in 1982 for "nonmedical" purposes, the commission reported.

Although no more recent figures were given on numbers of users, the report said the number of dosage units consumed increased from 3.03 billion in 1982 to 3.06 billion in 1984.