Duvalier, who made a surprise return to the Caribbean nation in January, was admitted Wednesday night, companion Veronique Roy said as she took a break from visiting him at the Canape Vert hospital in Port-au-Prince.
Roy declined to discuss his condition in detail. Asked if it was serious, she replied only, "I hope not."
Enzo Alcindor, a Duvalier family friend who was with them at the hospital, said doctors planned more tests. He quoted a cardiologist as saying Duvalier should be released after two days.
"It's not a stroke. It's not a heart attack. Thank God," Alcindor said.
Two police officers stood outside his hospital room Thursday evening.
The 59-year-old former dictator made an abrupt return to Haiti in January after 25 years in exile and appeared at times to move with difficulty, sparking speculation that he was ill.
He has been living in a villa in the hills above Port-au-Prince under police guard as a judge investigates whether he can be charged with a long list of crimes, including corruption and torture, committed while he was "president for life" in the impoverished Caribbean nation.
There have been no restrictions on his movement and he has been spotted attending a jazz concert in Petionville and has been receiving a stream of visitors at the house.
Roy and others said a court clerk served an order on Duvalier confining him to the villa but they planned to appeal.
Friends trickled into the private hospital to check on Duvalier, while outside a dozen sympathizers showed up to lend support, unfurling a banner with the Duvalier colors of black and red. It read "welcome back."
"When you're sick, anything can happen," said Exentus Belot, a 40-year-old mechanic. "He has been under so much pressure and that could have been a factor in his health."
Duvalier was ousted in 1986 amid a popular uprising against what was widely considered a brutal and corrupt regime. He assumed power in 1971 at age 19 following the death of his notorious father, Francois "Papa Doc" Duvalier.