McCain receiving treatment for side effects of cancer therapy

Sen. John McCain of Arizona, who is undergoing treatment for an aggressive form of brain cancer, is "currently receiving treatment at Walter Reed Medical Center for normal side effects of his ongoing cancer therapy," his office said.

McCain, 81, who was diagnosed with glioblastoma multiforme over the summer, "looks forward to returning to work as soon as possible," his office added.

Sen. Lindsey Graham of South Carolina, one of McCain's closest friends in Congress, told ABC News today that he just received an update on McCain's health and that he's "resting in the hospital."

"He's receiving treatment for the side effects of the therapy. I feel pretty good about the way the treatment is affecting his underlying cancer. But the treatment has a downside. So he's tryin' to get rested up," Graham said. "I'm very confident he'll come back and continue to participate for a long time to come."

Graham said he's been in touch with McCain's wife, Cindy McCain, and is expecting to set up a time to speak with his colleague by phone.

"He's had a heck of a Senate schedule, and I hope he'll take some time to regroup, do a little rehabilitation on his leg and come back," Graham said.

Sen. Jeff Flake, the junior senator from Arizona, told ABC News today that he was also texting with McCain's wife last night and that he expects his colleague to be back at work soon.