Joseph Mannis said his client was therefore unable to be questioned by lawyers of his wife of 14 years as part of divorce proceedings.
In previous court documents, Hopper had said he hoped chemotherapy would be successful and that he could recover.
The film star, 73, filed for divorce from Victoria Hopper in January.
'Outrageous conduct'
In the papers, which state that Hopper weighs about 7st 2lb (45kg), he is described by his doctor, David Agus, as extremely weak and unable to have long conversations.
Being questioned by his wife's lawyers "could actually threaten his ability to survive the current health crisis", Dr Agus adds.
But he has approved an appearance by Hopper to receive a star on the Hollywood Walk of Fame on Friday because it would be good for his state of mind.
Last month, a judge ruled that Mrs Hopper must stay at least 10ft (3m) away from her husband after the actor said he wanted to eliminate unnecessary stress.
In previous papers filed in Los Angeles Superior Court, Hopper - who has been married five times - claimed his wife had engaged in "outrageous conduct" over the past year.
He alleges she has been difficult over visitation rights for their six-year-old daughter Galen.
Mrs Hopper, 42, meanwhile, said the actor threatened her, smoked marijuana around their daughter and that his illness had left him unable to make sound decisions.
She has also claimed that he filed for divorce so that he could cut her out of her inheritance.
The actor, best known for films including Easy Rider, Apocalypse Now and Blue Velvet, has denied all the claims.
