A major international study of a new way of treating HIV has been stopped because researchers found it was doubling the risk of illness or death.
Patients taking part in the study were at far greater risk of illness
More than 5,000 people in 33 countries were given anti-retroviral drugs only when their immune systems weakened.
But they were twice as likely to die or suffer illness as patients on the drugs full-time, said the National Institute of Allergy and Infectious Diseases.
It had been hoped the treatment would minimise side-effects, and save money.
Investigators from the institute, NIAID, said that as well as the higher risk of death or illness, those taking part in the study also experienced "an increase in major complications such as cardiovascular, kidney and liver diseases".
NIAID recommended that doctors participating in the international study should resume full-time anti-retroviral therapy (ART) for all patients.
The findings by the US government are a blow to advocates of the study, who were trying to reduce the huge cost of ART, especially in poor countries where Aids is most widespread.
Some earlier small-scale studies had indicated that carefully monitored breaks in ART could keep Aids at bay.
The trial, known as Strategies for Management of Anti-Retroviral Therapy, has been running since 2002.
Nick Partridge, chief executive of the HIV charity Terrence Higgins Trust, said: "This trial has given us a crucial insight into how HIV should be treated in future.
"However, the results will be a disappointment for many people with HIV.
"Continuous treatment is complex, can cause side effects and can be difficult to stick with.
"At the moment, many people take a ┐treatment break┐ to improve their quality of life.
"It's essential that the information from this trial is disseminated as soon as possible.
"People living with HIV and their consultants need the latest information in order to make educated choices about future treatment regimes."