Sen. John McCain (R-Ariz.) makes his way to a meeting in Washington about the tax bill on Dec. 1. (Photo by Bill O’Leary/The Washington Post)

Sen. John McCain (R-Ariz.) has been admitted to Walter Reed National Military Medical Center for recovery from the side effects of another round of treatment for brain cancer, according to his office.

McCain, who missed a third straight day of Senate votes Wednesday, has been undergoing rounds of chemotherapy and radiation to treat glioblastoma, the terminal form of brain cancer he was diagnosed with in July. McCain has been undergoing treatments since early September at the National Institutes of Health in Bethesda, next to Walter Reed.

Two friends close to McCain, requesting anonymity to speak about his condition, said there were no plans for the senator to resign. In a statement issued from his office, McCain hopes to return “to work as soon as possible.”

Republican leaders expect to hold a vote early next week on the sweeping tax-cut plan that is currently in a House-Senate negotiation. The Senate version initially passed with 51 votes. If McCain missed the vote on final passage, Republicans would not be able to afford a single additional defection or absence to pass the legislation.

In recent weeks the senator, 81, has been increasingly debilitated from the side effects of what friends have said are increasingly difficult rounds of treatment. He suffered an Achilles’ tendon tear in early November that put him in a walking boot, and in recent weeks he has used a wheelchair to get to and from his office and the Senate floor for votes.

“Senator McCain is currently receiving treatment at Walter Reed Medical Center for normal side effects of his ongoing cancer therapy,” his office said in a statement. “As ever, he remains grateful to his physicians for their excellent care, and his friends and supporters for their encouragement and good wishes.”