The number of Americans using cocaine has remained stable in the last five years, but cocaine-related deaths and illnesses are rising as a result of a new fad that involves smoking, or freebasing, the drug and the availability of purer forms of the substance, federal experts said yesterday.

The increasing medical emergencies associated with cocaine, the experts added, are also the result of a number of longtime users beginning to feel the repercussions of drug abuse.

"We are picking up people who are falling off the cliff from years of cocaine use and who are using newer forms of the drug that are likely to produce greater addiction and side effects," said Dr. Donald MacDonald, administrator of the Alcohol, Drug Abuse and Mental Health Administration at the National Institutes of Health.

There has been growing public attention on cocaine because of increased arrests, lower drug prices and the recent cocaine-related deaths of athletes such as Maryland basketball star Len Bias and Cleveland Browns defensive back Don Rogers. But MacDonald said a detailed nationwide survey shows that the actual number of users "may have peaked in 1979 or 1980 and now we are in a flat plane" at about 4.1 million persons. The public perception of a growing problem, the experts said, was fueled by the increasing number of deaths and illnesses.

The conclusions presented by MacDonald and other drug experts at a news conference here were from a National Institute on Drug Abuse survey polling about 8,000 persons six times between 1972 and 1982. The survey showed that cocaine use rose dramatically across the country between 1972 and 1979.

Drug abuse experts said that, based on preliminary data from a 1985 survey to be released next month, they believe the number of users last year will be about the same or only slightly higher.

Yet, cocaine-related deaths nationally rose from 195 annually to 600 between 1981 and 1985, according to separate National Institute on Drug Abuse statistics. And the number of patients being treated in emergency rooms for cocaine-induced illnesses during that same period rose from 3,296 a year to 9,946, according to the survey.

In Washington during that period, cocaine deaths rose from five annually to 61 and the number of cocaine-related emergency room visits rose from 157 to 765.

The recent notoriety of cocaine has heightened public awareness of the dangers of the drug, which for many years was expensive and associated by some people with a glamorous, affluent life style. Many of those trying the drug in the 1970s believed it was nonaddicting.

The drug is now considerably cheaper to purchase, according to drug experts, but medical authorities in recent years have increased their warnings that it is highly addictive and that its effects can be deadly.

Doctors report that cocaine use increases the user's pulse rate and blood pressure. It also restricts the coronary arteries so that the amount of oxygen going to the heart is limited. In some cases, it can cause convulsions, stroke, heart attack and death.

MacDonald said new ways of using the drug that involve smoking especially pure forms of cocaine known as freebase cocaine or "crack" are partly responsible for the rise in the numbers of cocaine-related deaths and emergency room visits.

According to the National Institute on Drug Abuse, a survey of six major cities showed that about 2 percent of the persons treated in emergency rooms for cocaine overdoses in the first quarter of 1983 had smoked the drug, compared with 14.5 percent in the first quarter of this year.

Cocaine's physical effects are heightened with freebasing because it is more potent, MacDonald said. He said it also is more addictive than cocaine that is snorted.

"We think freebasing is potentially more dangerous" than snorting, he said.

Dr. Jerome Jaffe, head of the National Institute on Drug Abuse's addiction research center in Baltimore, said at the news conference that cocaine taken by freebasing "more rapidly hits the brain" and creates a feeling of an intense euphoria. That "high" is then followed by a severe low and an instant craving for more cocaine, which if satisfied causes the drug to build up in the blood stream, he said.

"Over a short period of time, you reach a higher toxicity," Jaffe said.

Dr. Michael Rosenthal, director of Phoenix House, a group of residential drug treatment programs with centers in New York City and California, said about 80 percent of the Phoenix patients in the past year have been treated for cocaine addiction, and of that number more than 60 percent were crack users.

He said that while it "takes years" for a typical cocaine user to become dysfunctional, "I've seen youngsters on crack in several months get so out of control they have to seek treatment."