Nigerians spent $1bn (Ј690m) on foreign medical trips in 2013, most of which was unnecessary, said Dr Osahon Enabulele.
Nigerian politicians were mostly treated by Nigerian doctors in the UK, he added.
Mr Buhari flew to London on Monday to be treated for an ear infection.
It is unclear where the 73-year-old would be treated for what his office described as a "persistent" infection.
Dr Enabulele, vice-president of the Commonwealth Medical Association, said it was a "national shame" that Mr Buhari went to the UK for treatment when Nigeria had more than 250 ear, nose and throat (ENT) specialists, as well as a National Ear Centre.
Mr Buhari should lead by example by using Nigerian doctors and facilities, and ensure government officials do not go abroad on "frivolous" medical trips, he added.
The UK had more than 3,000 Nigerian-trained doctors, and the US more than 5,000, Dr Enabule said, accusing the government of failing to address the brain drain by improving working conditions and health centres.
Mr Buhari, in a speech delivered on his behalf to the Nigeria Medical Association in April, said the government's hard-earned cash would not be spent on treating officials overseas, especially when Nigeria had the expertise.
Nigeria is one of Africa's biggest oil producers but most of its citizens live in poverty.
Mr Buhari took office last year on a promise to tackle corruption and waste.