On the assumption that crack cocaine creates more addicts and promotes more violence than powdered cocaine, federal law for a decade has dealt far more harshly with crack dealers and users than with others caught with more traditional forms of the illicit drug. Now, two psychologists specializing in drug addiction have determined that the physiological and psychoactive effects of different forms of cocaine are so similar as to make the existing discrepancy in punishment ''excessive.''

The study, by Dorothy K. Hatsukami at the University of Minnesota and Marian W. Fischman at Columbia University, appears in the current issue of the Journal of the American Medical Association. It breaks new ground by posing a medical challenge to the federal sentencing guidelines, which have been criticized over the last decade primarily for jailing more black offenders than white ones.

''The important issue is when possible to try to have science inform policy,'' Dr. Fischman said in an interview, explaining why the researchers undertook the project. ''Cocaine is cocaine. Regardless of whether you shoot it up or smoke it or snort it, it has the same stimulant effect.'' In their study, in which they examined laboratory experiments and other published studies on cocaine going back 20 years, Dr. Fischman and Dr. Hatsukami suggest sharply reducing the disparities in cocaine sentencing. But they stopped short of proposing that it be eliminated altogether.

An outbreak of crack-related crime in the 1980s prompted Congress to enact legislation in 1986 that punishes a first-time offender with five years in prison for possessing 5 grams of crack cocaine. To receive the same sentence, a first-time offender caught with powdered cocaine would have to possess 500 grams, or more than a pound. The researchers proposed reducing this 100-to-1 ratio to as little as 2 to 1. That would translate into the same penalty for possessing twice as much powdered cocaine as crack cocaine.

When cocaine was smoked in crack form, the researchers said, it still gave a faster, more intense high than powdered cocaine that was snorted, and therefore has greater potential for abuse. ''Crack cocaine is a lot easier to use, and it is more accessible to a broader population,'' Dr. Hatsukami said in an interview, explaining why she and her colleague believed there should be some difference in sentencing. ''We felt that crack cocaine does lead to more use.'' Dr. Fischman added, ''I feel that it is more dangerous because of its availability, not because of the drug.''

Cocaine hydrochloride, the scientific name for powdered cocaine, is refined from an alkaloid leached from coca leaves. It can be converted into crack by cooking the powder with baking soda and water in a microwave or conventional oven. Dr. Hatsukami and Dr. Fischman also concluded that treating offenders made more sense than locking them up. ''Although crack cocaine has been linked with crime to a greater extent than cocaine hydrochloride, many of these crimes are associated with addiction to cocaine,'' they wrote. ''Therefore, those addicted individuals who are incarcerated for the sale or possession of cocaine are better served by treatment than prison.''

Because most offenders jailed for crack have been black, the disparity has been attacked largely on racial grounds. According to a government study of illicit drug use nationwide, in 1993, 88.3 percent of federal defendants convicted of selling crack were black. By contrast, blacks accounted for only 27 percent of those convicted of selling powdered cocaine. In February 1995 the U.S. Sentencing Commission, a group of experts that draws up guidelines for federal courts, called the 100-to-1 ratio in crack sentences ''a primary cause of the growing disparity between sentences for black and white federal defendants.''

The commission recommended all but eliminating this gap. But Congress, with President Clinton's acquiescence, rejected the recommendation. The issue of the sentencing disparity has reached the Supreme Court, which ruled 8-1 last May 13 that statistics showing a predominance of black defendants for crack-related offenses did not constitute sufficient proof that the government selectively prosecuted African-Americans.

''It's a continuing problem,'' said Lloyd Epstein, a criminal lawyer in New York City who often represents defendants in drug cases. ''You find that black adolescents are arrested for crack on a daily basis and their sentences are more severe than whites or Hispanics arrested for powder.'' Under the current sentencing guidelines, Epstein said, a wholesale distributor of cocaine powder could receive a lighter punishment than his customer who bought a small amount of the powder and converted it into crack.

''The issue is not whether crack should be punished,'' Epstein said, ''but whenever you're dealing with justice, you're looking for some kind of parity.'' Julie Stewart, president of Families Against Mandatory Minimums, a group in Washington that opposes mandatory sentences for drug offenses, said, ''It's purely politics that prevents the right thing from being done on crack and powder.''

There is no sign that Congress plans to reconsider its tough stance against crack, particularly since there has been no public pressure to do so. ''When you mention crack cocaine,'' Ms. Stewart said, ''their minds immediately jump to an image of a young black man with gold chains and a gun in his pocket.'' The federal guidelines do not apply to cases tried in state and local courts. Only 14 states distinguish between crack and powdered cocaine in their own statutes, and these tend to involve larger amounts of drugs.