Liu Xia has been under house arrest since her husband won the award in 2010. She has never been charged.
Friends say she is depressed but fear that if she sees a doctor she might be sent to a psychiatric hospital.
Mr Liu was convicted of subversion in 2009. The Chinese authorities have never explained why they have restricted his wife's movements.
News of Liu Xia's condition came from fellow activist Hu Jia, a family friend and an outspoken dissident in his own right.
"Liu Xia was a very happy and cheerful person before Liu Xiaobo's arrest," Mr Hu told BBC Chinese.
"We all thought her sunny bright personality was the ideal complement for Xiaobo's more intense disposition."
But three years of house arrest had thrown Mrs Liu into deep depression, he said.
She is denied free movement or the right to receive visitors. Even her request of a daily walk in a local park had been turned down for fear supporters or journalists might approach her, Hu Jia said.
He said Mrs Liu felt guilty that her brother, Liu Hui, had been jailed earlier this year for fraud - she believed he had been prosecuted because of his connections to Liu Xiaobo and herself.
Hu Jia revealed that a sympathetic health professional had been prescribing anti-depressants for Liu Xia, but says he could not say for sure if she had taken any of the medication.
Hu Jia's wife Zeng Jinyan, who now lives in Hong Kong, has appealed on Liu Xia's behalf for "an internationally renowned psychologist from Medecins sans Frontieres" to be allowed to meet Liu Xia.
Hu Jia believes that this would be the only form of help that Liu Xia would feel safe accepting, but he doubts if the Chinese authorities would allow her such medical attention.
She also wants more access to letters from her husband, and the right to earn money.
Concern is growing for the mental health of the wife of jailed Chinese Nobel Peace laureate Liu Xiaobo.