Many African countries now have more doctors and nurses working in richer countries abroad than they have at home, research shows.
Previous studies have focused on medics trained in Africa
There has long been concern about the exodus of African medics, but the Human Resources for Health study suggests the problem may be greater than assumed.
Several countries, including Mozambique and Angola, have more doctors in one single foreign country than at home.
And for every doctor in Liberia, there are two working abroad.
The study, carried out by the Center for Global Development in Washington, looked at census records collected between 1999 and 2001.
It examined nine receiving countries: The UK, the US, France, Canada, Australia, Portugal, Spain, Belgium and South Africa.
The study is one of the first to count doctors who are born in Africa, not just those who are trained there.
Focusing on training location, the researchers argue, seriously underestimates the impact of losing people who want to become doctors has on a country's health service.
The report suggested the loss of doctors often went hand-in-hand with civil strife, political instability and economic stagnation.
NUMBER OF DOCTORS ABROAD
Mozambique - 75%
Angola - 70%
Ghana - 56%
Kenya - 51%
Rwanda - 43%
Sudan - 13%
Niger - 9%
Angola, Republic of Congo, Guinea-Bissau, Liberia, Mozambique, Rwanda and Sierra Leone all experienced civil war in the 1990s and all had lost 40% of their doctors by 2000.
Countries such as Kenya which experienced economic stagnation in the late 20th Century and Zimbabwe, which saw political repression as well as economic problems, saw more than half of their doctors leave.
At the same time countries with greater stability and prosperity, such as Botswana managed to keep many of their doctors, but so did very poor countries such as Niger.
The researchers speculated this could be to do with destitute countries not producing large numbers of would-be doctors with the financial capital or connections to leave.
The UK is one of the few countries to have introduced a code preventing it from actively recruiting from sub-Saharan Africa.
But despite this, Home Office figures show that 17,620 African doctors and nurses joined the NHS last year.
The Department of Health notes that while NHS trusts are banned from actively trying to enlist from Africa, there is little to stop health professionals from these countries applying for work permits to come to the UK.
The charity ActionAid said the brain drain was "a huge threat" to Africa.
"One of the best way to keep healthcare professionals in the countries that need them is to pay them properly - but currently health systems in many African countries are woefully underfunded," said Nick Corby, policy officer at the charity.
"The UK government could do Africa a real service by upping aid levels for health systems, ensuring that desperately needed doctors and nurses stay where the need is greatest."