Khodorkovsky declared a hunger strike last week, demanding that Vasily Aleksanyan, a former colleague suffering from AIDS, receive medical treatment.
"Khodorkovsky undergoes medical examinations every day," prison warden Valery German said, adding that Khodorkovsky has not indicated how long he is prepared to hold out with no food.
A court in Moscow refused last week to release from custody former Yukos vice president Aleksanyan, diagnosed with both AIDS and cancer. Khodorkovsky has demanded that Aleksanyan be transferred to a medical facility for urgent treatment, and alleges that authorities are withholding treatment until Aleksanyan confesses to charges.
Lawyers have repeatedly asked for hearings on Aleksanyan's case to be stopped and for him to be transferred to a medical facility for inpatient treatment. The former vice president of the now liquidated Russian oil company Yukos faces charges of embezzlement and money laundering.
Khodorkovsky is serving an eight-year prison term for fraud and tax evasion. He has consistently maintained his innocence calling the accusations politically motivated.
Once Russia's largest oil producer, Yukos collapsed after claims of tax evasion, which led to the company being broken up and sold off to meet debts. The bulk of its assets were subsequently bought by government-controlled oil company Rosneft.
Khodorkovsky's business partner Platon Lebedev, also serving his term for fraud and tax evasion, said Friday in a Chita court that he is ready to plead guilty to charges if it will help Aleksanyan.
The latest charges against Khodorkovsky and Lebedev, who were convicted in 2005, include stealing government shares, illegal oil trading, and laundering $25 billion earned from oil sales in 1998-2004. Both businessmen have denied the allegations, calling them politically motivated.
