The announcement prompted skeptical reactions and even the suggestion that it was a ruse to take him out of power.
"I don't remember official statements on Viktor Yanukovych's colds. But I remember well, when on Aug. 19, 1991, the vice president of the USSR, Gennady Yanayev, announced the serious illness of Mikhail Sergeyevich Gorbachev," political commentator Vitaly Portnikov wrote on his Facebook page.
Gorbachev's purported illness was reported as hardline Communists opposed to his reform efforts attempted an unsuccessful coup against him.
Yanukovych has faced two months of large protests, and authorities have so far failed to mollify the protesters.
In one of a series of moves aiming at resolving the crisis, the parliament this week voted for the repeal of harsh anti-protest laws. Yanukovych must formally sign that repeal and it was unclear whether he could do so while on sick leave. He also has accepted the resignation of his prime minister. But protesters say the moves are insufficient.
Yanukovych made a late-night visit to the parliament on Wednesday before it passed a measure offering amnesty to some of those arrested in two months of protests, but only if demonstrators vacate most of the buildings they occupy. The offer was quickly greeted with contempt by the opposition.
But the opposition regards the arrests during the protests — 328 by one lawmaker's count — as fundamentally illegitimate.
"Is this a compromise, or are these political prisoners," said 30-year-old Artem Sharai, demonstrating on Kiev's central Independence Square. "We will seize new buildings if the authorities don't really change the situation in the country."
Protesters are demanding Yanukovych's resignation, early elections and the firing of authorities responsible for violent police dispersals of demonstrators. The protests started after Yanukovych backed out of a long-awaited agreement to deepen ties with the European Union, but quickly came to encompass a wide array of discontent over corruption, heavy-handed police and dubious courts.
The bill would not apply to several city buildings in the center of Kiev which the protesters use as dormitories and operation centers, and are key support facilities for the extensive protest tent camp on the main square. With temperatures dropping as low as -20 Celsius (-4 Fahrenheit) during the night, continuing the protests without places to shelter would be virtually impossible.
But the Kiev city hall building, as well as regional administration ones seized by protesters in western Ukrainian cities, will have to be vacated, according to the Unian news agency.