"Today I found out I'm basically a dead man walking," said Ms Raich, a 41-year-old mother of two in Oakland, California.
As well as the brain tumour, she has scoliosis, chronic nausea and other medical problems.
She said she takes marijuana every two hours to ease her pain and boost her appetite and said she would continue to do so.
"Today the court said I don't have the constitutional right to basically stay alive," she said.
The three-judge appeals panel acknowledged that although support for the medical use of marijuana was growing, the US was not yet at the point where "the right to use medical marijuana is 'fundamental' and 'implicit in the concept of ordered liberty'."
Ms Raich said she would lobby Congress to change the 1970 Controlled Substances Act that bans marijuana.