Nigel Farage has said "terrible" shoulder and back pain meant he was not "firing on all cylinders" during the early weeks of the election campaign but insisted he is "back on form".
The Ukip leader revealed he was suffering after a flare-up of an old spinal injury and was prescribed strong sleeping pills and muscle relaxants.
Farage was dogged by speculation that he was ill during the early weeks of the campaign but said he was not unwell, but suffering from neuralgic pain.
The 51 year-old told the Daily Telegraph he has been receiving private hospital treatment in London and had been prescribed the sleeping drug Temazepam.
He said he may need medical treatment for the rest of his life.
Farage told the newspaper: "I was not unwell, I have not had heart palpitations, but I was getting increasingly terrible pain in my shoulder, my back, and so I was suffering from neuralgic pain.
"I am taking a few tablets but it is something I have got to live with, and I have got to pace myself. I think I am going to have medical treatment for the rest of my life."
But Farage admitted he had not been following his doctor's orders to the letter, leaving him seeming under par in the opening weeks of Ukip's campaign.
He told BBC News: "I've not been doing physio. I've not been doing my exercises.
"I've been so obsessed with the Ukip campaign that I put myself into a position where I wasn't really firing on all cylinders."
He added: "I'm not unwell at all but I did have neuralgic pain. I've sorted it out. I've reorganised my life."
The Ukip leader was injured in a plane crash on the day of the 2010 General Election, suffering broken ribs and other minor injuries.