The FDA has strengthened its warnings on the controversial antibiotic Ketek and banned the drug's use for treatment of sinusitis and bronchitis. The new move follows a yearlong FDA investigation into the drug's side effects.

"FDA has determined that the balance of benefits and risks for Ketek do not support continued approval of Ketek for these generally nonserious and often self-limited illnesses," said Dr. John Jenkins, director of the agency's Office of New Drugs.

The use of the drug will still be permitted for pneumonia.

The FDA investigation began after a scandal broke out over severe liver complications that led to the deaths of five Ketek patients. The deaths sparked a Senate Finance Committee investigation into whether faulty data and fraud were used in the FDA's approval process for the drug.

Under the FDA's new rules, Ketek will carry a "black box" warning against its use by patients with the neuromuscular disorder myasthenia gravis. The "black box" warning -- so called because of the thick black border used to make it stand out -- is the strongest warning that the FDA can put on a drug. Any stronger concerns about a drug's safety require that it be banned.

Even before the new rules, Ketek already carried a bold warning about possible liver damage. Stronger warnings have now been added about other possible side effects, including blurred vision and loss of consciousness.

Sen. Charles Grassley of Iowa, who headed the Senate investigation and has been highly critical of the FDA's approval of the drug, expressed satisfaction with the new rules. "When the spotlight was turned on the questionable way in which Ketek got approved ... the FDA was held accountable," he said.

But not everyone is convinced that the new rules go far enough. "A black box warning is the FDA's way of justifying the continued sale of dangerous drugs that should probably be pulled from the marketplace," said Mike Adams, author of "Natural Health Solutions."

"Every time the FDA can no longer refute the evidence that a particular drug is extremely dangerous to the public, it resorts to using the black box warning to deflect attention from the fact that the drug should probably be banned. It's a tactical ploy used to maximize Big Pharma profits, which seems to be the FDA's primary mission now."