Vice-President Nicolas Maduro told reporters after a mass for Chávez late on Friday that doctors began the treatment after the socialist leader recovered in mid-January from a respiratory infection that followed his December 11 operation in Havana.
The government said Chávez was flown back to Caracas on February 18 and has been at the capital's military hospital since.
Chávez has not been seen nor heard from since going to Cuba, except for a set of "proof of life" photos released on February 15.
He first revealed an unspecified cancer in the pelvic region in June 2011. He reported undergoing radiation treatment and chemotherapy after earlier operations.
Opposition leader Henrique Capriles accused Venezuela's government on Friday of repeatedly lying about Chavez's condition, and said the truth will be known within days.
Capriles tweeted the claim as Maduro assured Venezuelans on national TV that their cancer-stricken president continues a difficult and slow recovery after his surgery.
'Explain to the country'
"We'll see how they explain to the country in the [coming] days all the lies they've been telling about the president's situation," said Capriles, who Chávez defeated in October 7 elections.
Maduro accused opponents of spreading rumors about Chávez' health to destabilise the nation.
The opposition says Chávez should either be sworn in or declare himself incapable and call new elections. The constitution says he should have been sworn in on January 10, but Venezuela's Supreme Court said it was OK to wait.
Maduro attacked the Spanish newspaper ABC and Colombia's Caracol network for allegedly spreading lies about Chávez's condition. ABC said without specifying its source that Chávez's cancer had spread to a lung. It said he had been moved to an island compound in the Caribbean.
Chávez's son-in-law, Science Minister Jorge Arreaza, said on state TV that Chávez continues "to fight hard and is in the military hospital, as peaceful as he could be, with his doctors, with his family".
Arreaza is one of a small circle with access to Chávez, who the government says is battling a "respiratory deficiency" following a post-operative respiratory infection that required a tracheal tube.
Maduro blamed "the bourgeoisie", as the Chávez government refers to its opponents, of trying to destabilise the country and demanded they "cease the rumours". – Sapa-AP