A "dramatic" decline in the prescribing of longcontroversial antidiabetes pills was reported to the Food and Drug Administration by the Health Research Group yesterday.

Phusicians wrote 9.5 million prescriptions for the oral hypoglycemics in 1977, compared with 19.5 million 1973, said the HRG, which is financed by Ralph Nader's Public Citizen Inc.

The decline means that about 1.75 million diabetic patients have stopped using the drugs "despite the failure of the drug industry and FDA to adequately warn doctors and patients" that they are "dangerous and essentially ineffective," HR director Sidney M. Wolfe said in a letter to FDA Commissioner Donal Kennedy.

Wolfe urged Kennedy to warn an equal number of people still using the drugs that they "may be responsible for thousands of deaths a year" and, according to many studies, "are not even effective - for more than a short time at best - in lowering blood sugar."

The warning should be made directly, through "patient information sheets," rather than "merely informing those doctors who - despite all evidence to the contrary - have continued to use these harmful drugs," Wolfe said.

He issued the letter in connection with publication of a new HRG book, "Off Diabetes Pills: A diebetic's Guide to Longer Life," coauthored by Wolfe, Rebecca Warner, and Rebecca Rich.