Despite assurances that Buhari's health is nothing to worry about, the presidency is increasingly being urged to be more open about it.
The ailing head of state spent two months in London earlier this year, officially on vacation and to have routine medical tests for an undisclosed illness.
But he has missed two out of the last three cabinet meetings, was absent from Friday prayers last week and failed to attend his grandson's wedding on Saturday.
Buhari himself has disclosed he had blood transfusions and other tests in London and disclosed he "couldn't recall being so sick".
Aides, who during his absence maintained he was "hale and hearty", now say he had a "long period of treatment" in the British capital and needs rest.
His spokesman Garba Shehu said Buhari had spent most of his time at his private residence since returning from London in early March and was working from home.
Despite assurances that Buhari's health is nothing to worry about, the presidency is increasingly being urged to be more open about his condition.
"Why is the president hiding his state of health?" Nobel laureate Wole Soyinka said last week. "He's supposed to understand he's public property."
On Tuesday, a group of leading civil society activists urged Buhari take medical leave, as his absence "has fuelled further speculations and rumours" about the true state of his health.
"We are compelled to advise him to heed the advice of his personal physicians by taking a rest to attend to his health without any further delay," they wrote in an open letter.
A former chairman of Buhari's All Progressive Congress party, Bisi Akande, also expressed worries, saying "the health of the leader is intricately intertwined with the health of the nation".
"To avoid the ugly consequences of letting President Buhari's ailments throw Nigeria into confusion, I am urging all Nigerians to begin to pray for his divine healing and perfect recovery."
The health of Nigeria's president has been a sensitive issue since the death in office in 2010 of Umaru Musa Yar'Adua, which saw months of political turmoil.
Buhari dismissed claims during the 2015 presidential election that he was seriously ill with prostate cancer, saying it was an opposition smear designed to show him unfit to be head of state.