The unregulated supply of Aids drugs in the non-industrialised world threatens to accelerate the development of drug-resistant HIV strains.
By Richard Black
BBC Science correspondent
That is the conclusion of a study from the London School of Hygiene and Tropical Medicine, just published in the British Medical Journal.
Misuse of Aids drugs is common in Africa
The study urges governments and international agencies to deal with the problem now.
Drawing on evidence from Africa and Asia, the study shows that uncontrolled prescribing of anti-retroviral drugs is widespread and rising.
Where the state sector cannot or will not provide drugs, patients who can afford them naturally purchase where they can - from doctors, pharmacies, market sellers, or relatives abroad.
The study's author, Dr Ruairi Brugha, says that often patients do not take their drugs as they should.
"These drugs are not being used according to the correct regimens. For instance, monotherapy - just giving one anti-retroviral drug - is definitely bad practice. And we see evidence of that both from Zimbabwe and Uganda, and I'm sure it's happening in other countries too."
Dr Brugha also found that in some places patients are changing medication frequently, taking the wrong dose, or stopping treatment in periods when they cannot afford it.
This is exactly the set of conditions in which a virus quickly becomes drug-resistant.
Even in the rigid treatment patterns of the affluent west, HIV is becoming resistant to established anti-retrovirals - and this study says that governments and health authorities cannot afford to wait for more dangerous resistance to emerge in the developing world.
Doctors and clinics need treatment guidelines, they say, supplies of drugs need to be stable, and the public sector needs to compete more effectively in providing the services that people want.
A spokeswoman for UNAids confirmed that the problem is serious, and needs to be addressed urgently by scaling up structured treatment programmes.