Having to pay for HIV care increases the risk of treatment failure, a study by a leading aid agency says.
More than 40m in the world are living with HIV
The Medecins Sans Frontieres research found nearly half of people on drug treatment in Nigeria did not receive sufficient doses due to lack of funds.
The problem meant their immune system was much weaker than those on the free course run by MSF.
The agency said governments should be trying to ensure access to HIV treatment was free.
MSF studied 122 patients in Lagos who had enrolled for MSF treatment after previously having to pay for care.
The Nigerian government is insisting that public hospital patients pay for at least part of their own HIV care, including drugs and monitoring.
To pay for their care, 39% of respondents reported borrowing or begging, while one in five said they had been forced to sell property.
Dr Jens Wenkel, of MSF, said the patients who had taken insufficient drugs were at risk.
"These patients are more vulnerable to Aids-related infections such as pneumonia or TB.
"So forcing people to pay, and thereby risking treatment interruptions, can make this life-prolonging treatment less effective.
"If we want people to survive on treatment, we have to ensure access to free care."
Paying for HIV treatment is not uncommon. Other African countries such as the Democratic Republic of Congo also charge patients.
MSF provide anti-retroviral drug treatment for over 57,000 people in 29 countries.
Yusef Azad, National AIDS Trust's director of policy, said: "We still have a long way to go to reach the goal of universal access to treatment by 2010.
"Greater efforts need to be made to enable developing countries to provide free treatment, as well as improve their health infrastructure so that drugs can be delivered to the people who need them.
"The provision of free treatment is also a vital aspect of HIV prevention as people have little incentive to test if they cannot afford life saving treatment."