Anti-HIV drugs may come on complex prescriptions
Failure to follow instructions increases the risk that treatment will fail. It also raises the possibility that the virus will become resistant to the drugs.
They found that only 47% took their drugs as directed all the time.
The drugs that are prescribed to HIV patients are known as antiretrovirals.
They are prescribed on a complicated schedule, often requiring multiple doses two to four times a day, and may be linked to special dietary requirements.
Difficulty
The researchers said: "The finding that a substantial number of patients did not succeed in taking (the drugs) as prescribed illustrates the difficulty of consistently taking antiretroviral medication according to all requirements.
"To date, it is not known what level of adherence to (the drugs) is precisely needed to prevent viral rebound and the emergence of drug-resistant virus variants."
The researchers said the consequences of not following directions will vary with the medications involved.
They urged doctors to consider a patient's ability to follow the prescribed course when choosing among various treatments.
They also warned that because HIV-infected people could transmit a drug-resistant virus, the matter has "significant public health implications".
New drugs
HIV charity the George House Trust said the study was carried out in 1998 and 1999 when anti-retroviral drugs were very new, and some of the first regimes were more complex than they are now.
Director Denise McDowell, said: "People are not lab-mice.
"We have got to recognise that its very hard to change your lifestyle to fit the very complex regimes that many anti-HIV drugs require.
"It is vital that doctors don not just dispense medication without spending time with people to work out which therapies and life changes are possible, on an individual basis."
Ms McDowell said voluntary organisations could also play an important role giving the social and practical support many people need to sustain complex drug regimes.
John Godwin, head of policy and advocacy at the National Aids Trust, said research has shown that transmission of drug resistant HIV is increasing in the UK.
He said: "A UK study conducted at the University of Birmingham has shown that the estimated prevalence of drug resistance in those infected in 2000 was 27%.
"This has alarming implications. NAT wants to see minimum national standards put in place for patient services which help people adhere to these complex drug regimens."
The research is published in the journal Archives of Internal Medicine.