A "drastic decrease" in law enforcement actions by the Food and Drug Administration since the Reagan administration took office is threatening public health and safety, a Ralph Nader-founded consumer group charged yesterday.

Using the latest figures supplied by the FDA, the Health Research Group calculated that total enforcement actions through the first half of fiscal 1982 dropped by two-thirds under the Reagan team, as compared with activity during equivalent periods during the Carter term.

The group warned that the drop in government "policing" is an "open invitation to drug companies and food companies to violate federal law, thus risking the health and lives of 230 million Americans."

FDA spokesman William Grigg did not dispute the HRG's contention that enforcement actions had declined, but he emphasized that there had been a "conscious decision to obtain corrective actions voluntarily, to persuade a company to withdraw a product or not to distribute rather than going ahead with seizures and injunctions that cost the government more time and money."

The Health Research Group analysis includes seizures, injunctions, prosecutions, regulatory letters and other enforcement actions followed by the FDA's own computerized Legal Action Monitoring System.

Agency officials maintain the raw enforcement figures can be interpreted in various ways and figures within FDA often vary within different record-keeping branches. "If we were a police department and we had these figures the agency might be praised because lawbreaking was down," added the spokesman.

But Dr. Sidney Wolfe, the HRG director, and attorney Allen Greenberg attribute the declining enforcement numbers to an administration "partnership" with industry.

In a letter to House Energy and Commerce Committee Chairman John D. Dingell (D-Mich.), the group also raised concerns about the following, and Dingell promised to investigate.

* Hazardous information about the arthritis drug Oraflex, which has been withdrawn from the market because of mounting side effects and deaths, was withheld from the public by the FDA and Eli Lilly & Co. FDA declined to comment yesterday.

* The Justice Department has not prosecuted SmithKline Corp. despite an FDA recommendation in the spring of 1981 that criminal charges be brought in connection with suppressed data on side effects involving the banned anti-hypertension drug Selacryn. The FDA said it was satisfied that the case is "progressing."

* A warning label on aspirin about the risks of a life-threatening complication called Reye's syndrome in children with flu or chicken pox has not yet been proposed, despite Health and Human Services Secretary Richard S. Schweiker's announcement last June that such labeling would be required. The FDA says the aspirin warning label proposal is moving into final review, although opposition by the manufacturers may make it difficult to follow a "quick timetable.