Sen. John McCain John Sidney McCainMcSally says current Senate should vote on Trump nominee Say what you will about the presidential candidates, as long as it isn't 'They're too old' The electoral reality that the media ignores MORE (R-Ariz.) has injured his Achilles tendon and will wear a boot while recovering, his office said Monday.

“Senator McCain was treated at Walter Reed Medical Center over the weekend for a minor tear in his right Achilles tendon, as well as for other normal and non-life-threatening side effects of cancer therapy," McCain's office said in a statement.

They added that McCain "has returned to work in the Senate and will be wearing a walking boot until his injured tendon is fully healed.”

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McCain has been receiving treatment for brain cancer after announcing in July that he had been diagnosed with glioblastoma, an aggressive form of the disease.

His office didn't specify on Monday what "other normal and non-life-threatening side effects" the GOP senator had been treated for.

McCain joked about the latest side effect from his treatment on Monday, posting a photo of himself wearing the walking boot.

"I can't tell you how much I hate wearing this boot!" he said via Twitter.

I can't tell you how much I hate wearing this boot! https://t.co/W6zClDRpFb pic.twitter.com/x3mDC4n11H — John McCain (@SenJohnMcCain) November 6, 2017

McCain has emerged as one of his party's most vocal critics of the Trump administration's policies, including knocking "half-baked, spurious nationalism" and "propaganda and crackpot conspiracy theories" in a pair of recent speeches.

He was also one of three senators who helped kill ObamaCare repeal legislation in both July and September. He is considered a key vote on the GOP's tax-reform bill.