Radia Nasraoui has been refusing food since 15 October in protest against police harassment and surveillance.
Her lawyer, Fethi Touzri, a member of the committee watching over her medical condition, said the 49-year-old activist had lost seven kilos (15 pounds) since starting her strike and her blood pressure was irregular.
Quoting from a report he and two colleagues had produced, Touzri said on Saturday that Nasroui was displaying "great weakness, muscular problems, and difficulties with sleeping and memory".
Nasroui, wife of opposition leader Hamma Hammami, is demanding an end to police surveillance of her office and home, saying her mail was being opened and her telephone was tapped.
She has said permanent police shadowing made daily life impossible, and that "terror" had been exerted on her family and friends.
Nasroui says she is determined to continue her strike.
"It is the only recourse left to me to demand my dignity as a lawyer and a citizen," she told visiting reporters.
She has accused the authorities of wanting her "reduced to silence" and causing her isolation because of her human rights advocacy and denunciation of torture.
The Tunisian government has come in for frequent criticism - notably from abroad - for its repression of rights and frequent use of torture.
Nasraoui staged a month-long hunger strike last year to demand the release of her husband, sentenced in absentia in 1999 for membership of the banned Tunisian Communist Workers Party.
The 51-year-old opposition activist, who went into hiding at the time, emerged in February 2002 and was immediately arrested to serve a nine-year jail term, later reduced to three years and two months, before being released conditionally last September.