Cracking down on makers of “herbal fen-phen,” the Food and Drug Administration warned consumers Thursday that some products sold as substitutes for recently banned diet drugs may be dangerous.

The FDA wrote to a Pennsylvania company this week that its products violate federal law and may be seized as illegal drugs.

The diet drugs fenfluramine and its close cousin Redux were banned in September after doctors discovered the medicines could damage dieters’ heart valves. Since then, dieters have flocked to blends of dietary supplements that bear such names as Herbal Phen-Fen and Herbal Phen Fuel.

There is no evidence that the dietary supplements contained in such remedies, mostly ephedrine and St. John’s wort, are effective for weight loss, the FDA said.

Worse, the agency said, ephedrine can cause high blood pressure or even heart attacks and strokes.

The FDA said another herbal fen-phen product, 5-hydroxy tryptophan, is related to a dietary supplement banned for its link to a fatal blood disorder.