The Food and Drug Administration's system for detecting serious side effects of new drugs is so slow and cumbersome that it takes weeks to classify a reaction as serious and further weeks to enter the finding into a computer so the FDA is aware of the magnitude of the problem, according to testimony yesterday before a House Government Operations subcommittee.

Less than a month before the popular prescription pain reliever Zomax was voluntarily removed from the market, its manufacturer informed the FDA of 908 allergic reactions to the drug, according to an FDA memo.

Robert Eaton, the pharmacist tracking reactions for the FDA, testified that this was "double what I thought." He said that the discrepancy apparently resulted from a lag time in classifying and getting the reports into the agency's computer system.

The disclosure came yesterday at hearing at which the overall adequacy and speed of the government's system for keeping track of serious side effects once a drug goes on the market were sharply questioned.

The system is a crucial part of the Reagan administration's new effort to speed up approval of new drugs but monitor them more closely after approval.

Zomax was temporarily pulled off the market by the Johnson & Johnson subsidiary, McNeil Pharmaceutical, in March, following reports of five deaths and over 1,000 allergic reactions since the drug was approved for relief of pain in late 1980.

Subcommittee Chairman Ted Weiss (D-N.Y.) yesterday said, "FDA for whatever reason did not have its record-keeping up to date." He also cited statistics showing an overall drug backlog of "several weeks, if not months" to get adverse drug reactions classified as serious into the computer, as well as several weeks to get them classified in the first place.

"The length of time doesn't give me a sense of satisfaction or reassurance that FDA is keeping on top of the picture," Weiss said.

He also said that FDA figures show, for example, that in the first three months of this year three-fourths of the most serious and unexpected reactions took more than 30 days to be recorded in the computer and nearly 90 percent of known serious reactions took more than 75 days.

Dr. Judith Jones, head of the FDA's drug experience program, said, however, that the recent slowdown was "not representative" and resulted from temporary "management changes." She said that most of the unexpected serious reactions that occurred last year were entered into the computer within 30 days after classification.

FDA Commissioner Dr. Arthur Hull Hayes Jr. defended the FDA's post-market surveillance system as "the best system in the world."

Also questioned at the hearing were the agency's cancer policies, particularly the decision to approve Zomax in light of animal studies showing that the drug can cause tumors in rats. Agency officials defended the drug as having special pain-killing benefits and said the labeling adequately cautioned about the dangers of long-term use.

The FDA said that the drug now faces several barriers in returning to the market. But McNeil official Robert Z. Gussin yesterday said that labeling could direct the drug toward a special patient population, those with moderately severe chronic pain who need an alternative to narcotic drugs. For them, he said, the drug "offers effective pain relief at an acceptable risk."

Gussin was accompanied by a group of doctors who argued that Zomax is needed for their patients. Dr. Ellen Bernstein, a New Jersey eye surgeon, said that she personally needs the drug to treat the pain of spreading breast cancer "with no impairment of functioning."