Drug dealers are substituting diazepam, or Valium, for Quaaludes in dangerous dosages and selling it to gullible drug users at the same high prices as the Quaaludes, according to the Drug Enforcement Agency.

The dealers are substituting diazepam for Quaaludes in dosages that average eight times the strength of prescription dosages for Valium. Some of the diazepam tablets seized by the DEA in recent months have a dosage strength as much as 29 times that of prescribed Valium, a stronger dose than has ever even been tested on animals.

"Nobody knows what these doses of diazepam can do to people," Gene R. Haislip, director of DEA's Office of Compliance and Regulatory Affairs, said in an interview. "We don't know this for sure yet but we think a lot of the traffic accidents and emergency room visits being blamed on Quaalude overdoses are really due to diazepam."

In the last few years, Haislip said, the DEA has seized more than 7 million tablets that turned out on chemical analysis to be diazepam instead of Quaaludes. Half of a huge number of contraband Quaaludes seized a few months ago in Los Angeles County were diazepam.

Canadian police recently seized 4,400 pounds of diazepam powder and 1.4 million diazepam tablets disguised as Quaaludes. Haislip said the Canadian dealers were shipping as many as 12 million of the counterfeit Quaaludes to a Miami drug ring every month.

"Disguising diazepam for Quaaludes may be the fastest growing illegal drug business in the country right now," Haislip said.

What worries Haislip the most is the dosage strength of the diazepam.

Little clinical knowledge exists on the effects of Valium at such strong doses on people. East European doctors tried higher doses of Valium (60 to 70 milligrams) on psychotics and gave up on the higher doses when they found it made their patients even more psychotic.

"We now know that Valium even at standard doses stays in the bloodstream three or four weeks after being administered," one psychiatrist who asked not to be named said. "At the very least, these stronger counterfeit doses will stay with the user a longer period of time and make him more susceptible to things like Valium addiction. It is not nice to contemplate."

Besides Canada, the source of most of the illegal diazepam being smuggled into the United States is Mexico and Colombia.

In 1981, narcotics agents seized 58 tons of illicit methaqualone from Colombia alone, enough to make 180 million Quaalude tablets with an average street value of $4 each on the street, a total of $720 million.

Until about a year ago, dealers from all countries were smuggling more than 400 million tablets of Quaaludes into the country every year, which on the street is almost a $2 billion-a-year business, Haislip said.

"To give you an idea of how big this illegal traffic had grown," Haislip said, "less than four tons of methaqualone were used in legitimate prescription tablets last year."

But in the last year, the DEA in cooperation with narcotics agents in Germany, Austria and Hungary has made several hundred arrests in the Quaalude trade, including "several dozen" key figures.

Said Haislip: "We believe we've shut off 90 percent of the counterfeit Quaalude business. That's very close to complete success."

The problem is that illegitimate diazepam is now coming into the United States the same way the illicit methaqualone was coming in two years ago, he said.