It was a bold promise. One customer claimed a single dose of a hangover remedy called Drinkin' Mate made him "feel better in just minutes. Soon enough, my entire hangover was gone." But such testimonials ran afoul of the U.S. Food and Drug Administration, which recently concluded that the Plymouth-based maker of Drinkin' Mate can't claim medical benefits from an unapproved drug and told the firm to either seek approval for its berry-flavored, effervescent tablets or adjust its marketing. Frederick Wehling, president of Amerilab Technologies Inc., said his Minnesota company has already addressed the problem by adjusting its labeling and removing such testimonials from its website. "It was an oversight on our part,'' he said.