Rights activist Mohammadi, 52, has been jailed since November 2021, and has spent much of the past decade in and out of prison.
A group of supporters of Mohammadi, who in 2023 won the Nobel Peace Prize in recognition of her advocacy work, said they had been informed of the results of medical tests carried out last month "which showed a worrying deterioration of her health".
"The Free Narges Coalition is extremely worried about the deterioration of Narges Mohammadi's health in detention," the group said in a statement, noting cardiovascular, gastrointestinal and other risks.
Mohammadi, who is held in Tehran's Evin prison, should be released "immediately" and have access to medical care "without delay", the coalition added.
In the past eight months, Mohammadi has been suffering from acute back and knee pain, including a herniated spinal disc, the supporters said.
Mohammadi has kept campaigning even behind bars and strongly supported the protests that erupted across Iran following the September 2022 death in custody of Mahsa Amini who had been arrested for allegedly violating the Islamic republic's strict dress rules for women.
In recent weeks, Mohammadi and other women held with her at Evin have staged protests in the prison yard against death sentences handed to two Iranian Kurdish activists, Pakhshan Azizi and Sharifeh Mohammadi who were tried for membership of an illegal armed group.
Narges Mohammadi received in June a new one-year prison term for "propaganda against the state", on top of a litany of other verdicts that already amounted to 12 years and three months of imprisonment, 154 lashes, two years of exile and various social and political restrictions.
