Aldo Ceresa, who took part in the trial, first noticed problems differentiating between left and right 10 years ago.
After his diagnosis, the 68-year-old, who is originally from Glasgow and now lives in Oxfordshire, close to his family, had to give up his job as a surgeon.
Mr Ceresa took aducanumab for two years before the trial was halted - and then had to wait almost as long for another trial, at the National Hospital for Neurology and Neurosurgery, in London, to begin.
"I'm quite happy to volunteer," he says.
"I really, really enjoy this journey that I'm going through - and obviously the benefits I'm getting from it, which I'm very, very grateful for."
He is convinced the drug has helped him.
"I feel like I'm not quite as confused. Although it's still there, it's not quite as bad.
"And I'm just getting that bit more confident now."
Mr Ceresa says his family has noticed improvements too.
"Before, if I was going to get something, I couldn't remember, you know, where to find things in the kitchen.
"That has become less of a problem," he says.
"I haven't caught up to the level that I was before - but I'm heading in the right direction."
And Prof Robert Howard, professor of old age psychiatry at UCL, went further calling the drug's approval "a grave error" that could derail the ongoing search for meaningful dementia treatments "for a decade".
He said the FDA had ignored data from the trial which showed no slowing of decline in cognition or function.
However, Alzheimer's Society said the drug was "promising" but added it was "just the beginning of the road to new treatments for Alzheimer's disease".
Another charity, Alzheimer's Research UK, said it had written to the Health Secretary Matt Hancock asking the government to prioritise the fast-track approval process for the drug in the UK.
Chief executive Hilary Evans, said: "People with dementia and their families have been waiting far too long for life-changing new treatments.
"It is now essential that regulatory authorities assess the evidence to decide whether they believe the drug is safe and effective for use in the UK."
Although many doctors are doubtful of aducanumab's benefits, its US approval could be a huge boost to dementia research, which is traditionally underfunded compared with cancer or heart disease.