Using the military to intercept illegal drugs entering the United States, as proposed by Congress, would neither greatly raise street prices nor reduce the amount reaching the country, a new study says.

"There are dozens of ways of bringing drugs into the country, and only a few of them can be effectively blocked by the military," said Rand Corp. economist Peter Reuter, main author of the 154-page report by the California think tank."Smugglers adapt," Reuter said in an interview Tuesday. "As interdictors make risks and costs higher, smugglers shift to other methods of bringing drugs into the country."

The House has passed a far-reaching amendment to the $299.5 billion Pentagon appropriation bill ordering President Reagan to have the military basically seal off U.S. borders against illegal drug-trafficking.

The Senate passed a slightly less sweeping version but still gives the military a bigger role in stopping the flow of illegal drugs into the country. It would give new arrest powers for Navy officers aboard warships that stop suspected drug boats on the high seas.

Differences between the two bills will be resolved in a House-Senate conference committee.

Defense Secretary Frank C. Carlucci and other administration officials have opposed using the military in the war against drugs.

Reuter provides support for their position, saying his study found that use of the armed forces against drug smugglers has had little effect so far, despite big increases in expenditures.