A spokesman for the Russian prison service, Aleksandr Sidorov, said Khodorkovsky had suffered stomach problems after coming off a two-day hunger strike to protest against being placed in solitary confinement.
He said Khodorkovsky's health was not at risk and that he would soon be returned to the one-man cell where he was moved in April after a fellow inmate slashed his face.
A lawyer for Khodorkovsky, Natalia Terekhova, refused to comment other than to deny that her client was suffering from food poisoning.
Earlier in the day, a Moscow court turned down an appeal by Khodorkovsky against his sentence.
Khodorkovsky's lawyer, Genrikh Padva, told reporters today the court had informed the defense team that it saw no grounds for reconsidering the eight-year prison sentence.
Padva said Khodorkovsky's defense had already been lodged another appeal with Russia's Supreme Court.
The trial against him was widely seen as punishment by Russian authorities for his political ambitions and part of a drive by the state to regain control over the key oil industry.