Longtime National Rifle Association foe Frank Lautenberg, the New Jersey senator, missed Thursday's critical gun control vote, raising concerns about his illness that has kept him from the Senate chamber.

Lautenberg has been absent recently under doctor's orders that he remain at home while he receives treatment for fatigue and muscle weakness. He also recently had the flu.

The 89-year-old Lautenberg underwent chemotherapy treatments for stomach cancer in 2010, but an aide dismissed rumors that the cancer has returned. He recently announced that he will retire and won't run for re-election in 2014.

Despite his absence, Lautenberg has remained busy. Just this week, for example, he reintroduced his Safe Chemicals Act aimed at protecting Americans from toxic chemicals in consumer products.

His office provided Secrets a statement that said in part, "I regret that I will not be returning to Washington next week as I continue treatment for, and recuperate from, muscle weakness and fatigue. My physician continues to advise me to work from home and not travel at this time."

Lautenberg added: "I am disappointed I will not be present for the opening of the debate on gun legislation in the Senate. It is an issue I am deeply passionate about, and my victories over the gun lobby are among my proudest accomplishments. I am, however, gratified that my legislation to ban high-capacity ammunition magazines will be one of the key amendments offered to this bill."