Weir announced in June he was suffering from the degenerative condition to raise awareness of the condition.
However, in an interview with BBC Radio Scotland’s John Beattie, a fellow former Scotland rugby player, Weir, 47 said he doubted rugby was the cause of his MND, despite not being the first rugby international to develop it.
Some researchers suggest repeated head injuries can increase the chances of developing the disease.
Admitting the thought had occurred to him and he had asked his medical professor about it, Weir said: “We have maybe 450 to 500 sufferers of MND in Scotland - and quite a large majority probably wouldn’t have played rugby.
“I don’t think there’s any connection to it at the moment but the reason behind that is that there’s maybe not a lot of studies put together, not a lot of work or focus put on MND at the moment in the world, so hopefully we can try and change that and try to find the solution.”
Weir, who won 61 Scottish caps, has set up the Doddie’5 Discretionary Trust - which makes use of his rugby shirt number - to give practical and financial help to his family as his condition develops.
A foundation, My Name’5 Doddie, is being established to raise funds for research into a cure and to give grants to those with the condition.
Weir said he got a better understanding about MND through the J9 Foundation set up by fellow MND sufferer and former rugby international Joost van der Westhuizen, who died earlier this year.
“He probably brought me on to understand what it was all about. I met him at Murrayfield a number of years ago. A very inspirational character.
“So with his J9 Foundation it was really the first port of call that I was going to do, to see what he’s done before - because there’s a frustrating part with MND and that’s the solution. That’s not been found yet. It’s a terminal illness and that’s quite hard to take.
“You have to have a positive outlook - that’s what the J9 boys were saying. Somebody has put this card into my hands and with that, I have got to try to make a difference - and hopefully we do.”
However, Weir also spoke movingly of his fear of missing out on his three young sons, aged 16, 15, and 13, growing up.
He said worried he might not be able to drive them to places, give advice on girlfriends, or help with education and university, but said he and his wife were concentrating on building up happy family memories.