Young Americans have cut down their use of drugs such as marijuana and methaqualone (Quaalude), but more overdoses of cocaine and heroin are showing up in hospital emergency rooms because of changes in the ways these drugs are being used, according to government officials.

"The rapidly increasing epidemic of drug use in this country in the 1960s and 1970s has finally begun to recede, particularly among young people," Dr. Edward N. Brandt Jr., assistant secretary for health at the Department of Health and Human Services, said yesterday at a news conference.

"However, while there may be fewer illicit drug users in our population, we know from the people who show up in emergency rooms that the health consequences of drug use have not abated," he said.

In the first nine months of 1982, Brandt reported, 9,139 victims of heroin overdose were admitted to 820 hospital emergency rooms in 26 metropolitan areas monitored by the National Institute on Drug Abuse--roughly a one-third increase over the same period in 1981.

The same 820 emergency rooms took in 4,615 people suffering from cocaine overdoses in the first three-quarters of 1982, more than the total for all of 1981. The 820 hospitals sampled by NIDA constitute less than 20 percent of the nation's total.

A more in-depth sampling of emergency room cocaine cases in six cities by the Drug Enforcement Administration from May to October, 1982, showed dramatic increases over the same period in 1981. New York showed a 36-percent increase; Boston, 46.4 percent; New Orleans, 52.9 percent; Miami, 71.5 percent; Philadelphia, 83 percent, and Los Angeles, 90.4 percent.

The number of heroin- and cocaine-related deaths is also increasing, although the statistics for drug fatalities run at least nine months behind those for emergency room admissions.

Reports from 85 medical examiners showed 272 cocaine deaths in 1980, 335 in 1981 and a rising trend in 1982. The same medical examiners reported 898 heroin deaths in 1980, 930 in 1981 and a similar rising trend for 1982.

Most of the heroin overdose cases were addicts using a stronger form of the drug that has been available in recent years, Brandt said.

Most of the cocaine overdose victims were users who moved from the conventional method of snorting the adulterated powder through the nose to the more dangerous techniques of injecting liquid cocaine into the veins, called "mainlining"; injecting it with heroin, called "speedballing," or smoking it in its pure powder form, known as "freebasing."

"We're seeing a definite shift to a more vicious type of cocaine abuse," Gene R. Haislip, DEA's director of enforcement, said in an interview.

"There is a clear upward nationwide trend to mainlining and freebasing the drug that is an obvious change for the worse," Haislip said.

Freebasing cocaine gets the drug to the brain in its pure form in fewer than seven seconds, a method of use that narcotics experts describe as "devastating."

"There is probably no more of a compulsion for a drug than the freebasing of cocaine," said Dr. Jack Durell, associate director of NIDA for science, in an interview. "It very quickly becomes a devastating experience that drives the users into total submission."

On the brighter side, NIDA reported yesterday that its annual survey of high school seniors indicated that daily marijuana use by young people has declined for the fourth successive year.

NIDA's survey of 17,700 students across the country showed that one out of 16 high school seniors used marijuana daily. As recently as 1978, one in nine seniors smoked marijuana every day.

NIDA said it sees the same downward trend in young people's use of Quaalude and cocaine; NIDA officials said that DEA's crackdown on illegal methaqualone imports and the high price of cocaine ($100 a gram) could be factors.

Why are young Americans smoking less marijuana? Drug experts said there is increasing concern among youth about the effects of chronic use on their health, and less peer acceptance of marijuana use. The growing necessity for students to do well academically also is cited.