There are some worries in the Senate about a recently frail-looking Sen. John McCain, R-Ariz., as he deals with the side effects from his brain cancer treatment, CNN reported.

Unnamed Senate sources told CNN that McCain was less energetic in meetings — speaking up less often than usual – and frail-appearing before his week-long absence for treatment at Walter Reed Medical Center.

Sen. Lindsey Graham, R-S.C., McCain's closest friend in the Senate, told CNN he had spoken with Cindy McCain on Wednesday about her husband's health.

"He is receiving treatment for the side effects of therapy," Graham told the outlet. "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."

He said he hopes to talk to McCain on Thursday or see him in person soon.

"The main thing I am focused on is to just get better, John, take a little time," Graham said. "Rest up. It's OK. Take a day or two off. He's had a heck of a Senate schedule. And I hope he will take some time to regroup, do a little rehabilitation on his leg, and come back and participate in a way that only Sen. McCain can participate."

According to CNN, McCain was treated last month for a minor tear to his right Achilles tendon that required him to wear a walking boot.

The 81-year-old McCain is five months into treatment for brain cancer, CNN noted.