Seven months ago, Nomlindelo Nkoninga began giving her four-year-old son a combination of drugs to stem the progress of HIV. But only last month did he weigh enough to take the antiretroviral (ARVs) medication as three separate tablets, instead of the syrups commonly prescribed for young children.

"It's easier now because some syrups need refrigeration and I don't have a refrigerator," said Nkoninga, 24, who lives in South Africa's rural Eastern Cape province.

In recent years, many countries in sub-Saharan Africa have begun to distribute government-funded ARVs to some of the estimated 4.4 million people in need of the drugs. Yet, despite advances made in AIDS treatment for adults, it remains complicated and expensive to establish the correct doses of ARVs for children, who are smaller and constantly growing.

"A majority of people are not treating children, because they are scared of computing the doses," said Dr Eric Goemaere, head of Medecins Sans Frontieres (MSF) in South Africa.

MSF, which has launched a public campaign around this issue, claims that children with AIDS are needlessly dying because medicines have not been simplified for widespread dispersal. The medical humanitarian agency alleges that because most children with HIV/AIDS live in the developing world, there is little commercial interest in creating and marketing child-friendly treatments, and children are given small portions of adult doses.

"With kids, you have to individualise, and each one gets their own doses, their own regimens," said Dr Hermann Reuter, project coordinator of MSF's treatment programme at Lusikisiki in the Eastern Cape, where children make up eight percent of ARV users.

Although treatment for HIV is constantly evolving, generic ARVs for adults typically consist of three drugs in a "fixed-dose combination" in one pill. Most adults on ARVs therefore take standard doses of drugs in one or two pills twice a day.

The most common combination of drugs for first-time ARV users is "D4T," (or Stavudine, trade name Zerit) made by BrisolMeyersSquibb, "3CT," (or Lamivudine, trade name Epivir) made by GlaxoSmithKline, and "NVP," (or Nevirapine, trade name TK), made by Boerhinger Ingelheim/Roxane.

But treatment for children is not so simple. Pharmaceutical companies have not yet developed fixed-dose combination treatments in dosages appropriate for them, and physicians must often portion out a cocktail of three separate adult-dose medicines in different combinations as the child grows.

To determine correct paediatric doses most effectively, caregivers should ideally use the three drugs according to the surface area of the child - a number obtained by a complicated formula of multiplying the child's weight by its length, dividing by 3,600, and then taking the square root of that figure.

This kind of calculation is often impossible in the developing world. "We don't have calculators at the clinics and no one can work out square roots in their heads," said Reuter. "It's just not practical. If you're doing this in clinics, you have to simplify."

Simplification means setting dose standards - including combinations of syrups and crushed or broken pills - by the weight of the child. This sometimes results in overdosing HIV-positive children and increased side effects, Reuter explained, but overdosing is generally preferred to underdosing, which can gradually lead to resistance to the medication.

"As soon as there's not enough drugs in the body, the virus starts to multiply," Reuter said. "As soon as it multiplies, there's a chance for it to develop resistance and if there's resistance, the ARVs won't work any more."

Paediatric formulations are also more expensive than adult treatments. MSF noted that while the most popular fixed-dose combination pill (D4T, 3TC, and NVP) are available for adults at about US $200 a year, treating a 14-kg patient with three separate drugs costs roughly $1,300 a year.

"We need to make paediatric treatment nurse-friendly at a primary care level," said Goemaere. "We need to make it so that treating children becomes as simple as treating adults - close to home, in their own health centres, in their own environment, in their own language."