JUST WHEN EVERYONE IS JUMPING on the anti-drug bandwagon, there is a surprise disclosure concerning Supreme Court Justice William H. Rehnquist, the man likely - and qualified - to be the next chief justice of the United States.

It seems, according to a congressional medical report revealed in the Washington Post, that Mr. Rehnquist was a heavy user of Placidyl, a powerful sleeping pill, from 1977 to 1981. According to Dr. Freeman H. Cary, the Capitol physician who prescribed the drug when chronic back pain kept the justice awake at night, Mr. Rehnquist was taking three times the recommended dosage. Placidyl is usually recommended for only short-term use, say for a week or two. Reportedly, it took a hospital stay to break Mr. Rehnquist of the habit.It must be said that his dependence on the drug reportedly was broken in 1981, and that even during the period in question he performed as an able - some say superior - jurist.

This disclosure should not hamper Mr. Rehnquist's nomination to a position for which he is strongly qualified. Other attempts to prove him unworthy of the post have been discredited and this should not be used as a last-ditch effort to deny the conservative Mr. Rehnquist the chief justice's seat. But the news points out that the drug issue is much more complicated than is indicated by White House drug test gestures.

Contrast Mr. Rehnquist with the case of Brian Tribble, the friend of Maryland basketball star Len Bias. Mr. Tribble, charged with supplying Mr. Bias with the cocaine that probably killed him, has had to mourn his best friend while dealing with a criminal investigation that will probably end with his going to jail. We hold no brief for Mr. Tribble, except one of compassion. Our point is that the use of drugs, whether prescribed or illegal, is a problem in our society.

The nation needs to do some hard thinking about what its policy toward drugs will be, and the early signs of a tug-of-war between President Reagan and House Speaker Thomas P. O'Neill on the issue are not encouraging.

Drug policy should not be an attempt to pin one person's drug use on another, usually a friend who supplied the drug. On the other hand, habitual trafficking in illegal drugs deserves prolonged incarceration. Nor should our

drug policy permit those of wealth or position to be treated differently than those who are on a lower economic level.

Back in the 1960s and 1970s, when it became clear that many Americans - and some of them our children - were taking drugs, there was a good deal of worry that prosecution would be selective and political in nature.

We're not there yet, but it's a good time to be cautious. Maybe with Mr. Rehnquist on the bench, there will be more understanding on all sides.

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