Three weeks after the FDA allowed Wyeth to begin marketing what the drugmaker calls a reformulated version of its heartworm med on a restricted basis, a pair of dogged Congressional reps want the agency to justify its decision. ProHeart 6, you may recall, was yanked four years ago after being linked to an unusually high rate of deaths and serious side effects.

The episode spoke poorly of the FDA, though. In 2003, an FDA vet examined a growing number of adverse reactions, prompting the withdrawal. Wyeth then launched a secret investigation into the vet; raised conflict-of-interest charges against her at the FDA; and implicitly threatened FDA officials that further action would be pursued if the vet was allowed to remain in place. Among those involved in a failed effort to allow ProHeart 6 back on the market was former Wyeth ceo Bob Essner.

The vet, Victoria Hampshire, transferred within the FDA. Meanwhile, a 2005 FDA advisory committee met to evaluate the safety and risks associated with ProHeart 6 and recommended that more data, including targeted animal safety studies, was needed. However, the new Wyeth application for ProHeart 6 indicates the panel “did not require target animal safety studies for this supplemental approval.”

And so John Dingell, a Michigan Democrat who chairs the House Committee on Energy and Commerce and Bart Stupak, another Michigan Democrat who chairs the Subcommittee on Oversight and Investigations, have sent letters to the FDA demanding answers (one and two). Dingell, in fact, believes “there is simply not enough new data to justify reintroduction of this controversial product to the market.”

“Given the safety concerns raised in connection with the use of ProHeart 6 in dogs, shouldn’t CVM have convened a VMAC or other independent public forum to thoroughly evaluate the safety and risks of the drug before reintroduction to the market?” Stupak says in a statement. The Senate Finance Committee, by the way, investigated the Hampshire scandal. Here is the report.