Sen. John McCain was hospitalized again this week as he contends with an aggressive form of brain cancer and is reportedly not looking so healthy, but colleagues are optimistic that he will return to Capitol Hill soon.

The Republican from Arizona had missed votes Monday and Tuesday, and on Wednesday it was revealed he was "receiving treatment at Walter Reed Medical Center for normal side effects of his ongoing cancer therapy," according to a statement from his office.

Sources told CNN that McCain, who is 81 years old and was diagnosed in July with a primary glioblastoma, has been less chatty and not looking as healthy as he once did.

Sen. Lindsey Graham, a close friend of McCain's, said he spoke to McCain's wife, Cindy, on Wednesday for an update, which he indicated bore positive news.

"He is receiving treatment for the side effects of therapy," Graham, R-S.C., said, according to CNN. "I feel pretty good about the way the treatment is affecting his underlying cancer. But the treatment has a downside. So he is trying to rest up. I am very confident that he will come back and continue to participate for a long time to come."

Sen. Jeff Flake, McCain's Republican colleague from Arizona, echoed this sentiment.

"We expect him back," Flake said to reporters. "I expect him back. He's obviously recovering from the effects of the treatment and so we expect him back."

McCain's anticipated return to work could come at a critical moment as Republicans in Congress are working to finalize a major tax reform deal by next week and the GOP only has a razor-thin 52-seat majority in the upper chamber.

Meanwhile, Sen. Thad Cochran, R-Miss., has also missed votes this week. He had a non-melanoma skin lesion removed from his nose on Monday and his office indicated that he will be in attendance for votes next week. The 80-year-old lawmaker also dealt with recurring urinary tract infections during the fall.