Marathon runner Dewi Griffiths has been forced out of the Great Britain team for the World Athletics Championships in Doha because of a mystery illness.
The 28-year-old Welshman says months of unexplained tiredness and headaches stopped him training.
Griffiths was selected alongside Callum Hawkins after finishing 16th in two hours 11 minutes 46 seconds at this year's London Marathon.
"I've just been tired with no explanation for being tired," he said.
"You've slept fine, you've eaten fine, but physically you just can't run.
"I wasn't bedridden, but I'd have a headache that might last for a couple of days."
Griffiths was Britain's second-fastest current marathon runner behind Sir Mo Farah until Hawkins ran quicker in April's London Marathon.
It was during that race that things started to go wrong for Griffiths. Despite qualifying for his first World Championships, he said he did not feel right on the day.
Blood tests showed no problem and Griffiths says one doctor even suggested his problem was all in his head.
Griffiths struggled through his training programme and a few races before admitting defeat.
He is unlikely to race again in 2019, but aims to resume training in the next month before returning to competition in 2020 when he hopes to compete at the Tokyo Olympics.
Griffiths says the issues could have been a combination of a lingering virus and pushing himself too hard in training.
"I've gone into that red zone," he says. "It's just taking quite a bit of time to get back out of it.
"Hopefully now I understand what's going on [with my training] I can rectify it and get healthy again.
"I still have the belief I can do it. I've already run a 2:09 marathon so it's not doing something I've never done," hw said.
"The plan is to make the start line in Tokyo - it happens to be my birthday as well. So even more of a reason to make it."