By Kate Kelland

LONDON (Reuters) - Tens of millions of people in low and middle income countries would be pushed below the poverty line by buying common but vital medicines which are already unaffordable to hundreds of millions more, a study has found.

Laurens Niens' team at Erasmus University Rotterdam analyzed the number of people who would be pushed below an income level of $1.25 or $2 a day - poverty indicators used by the World Bank - by paying for four important, widely used medicines.

The Dutch researchers whose work was published in the Public Library of Science (PLoS) Medicine journal on Tuesday said their findings showed that greater effort is needed to encourage the use of cheaper generic drugs in poor countries and to ensure more medicines are made available through the public sector.

The drugs studied were a salbutamol inhaler, used for the management of asthma, glibenclamide, a common diabetes drug, atenolol, which belongs to a drug class commonly known as beta-blockers and is used to treat high blood pressure, and amoxicillin, a broad spectrum antibiotic.

Using data from the World Bank and the World Health Organization, the researchers generated "impoverishment rates" for these medicines in 16 low- and middle-income countries.

The results of their study show the much greater impoverishing effect of branded drugs which are sometimes the only option available in countries where public sector provision is often patchy.

In Yemen, for example, where seven percent of people live on less than $1.25 a day, buying branded glibenclamide - sold as Daonil by Sanofi-Aventis - would impoverish another 22 percent, but buying the cheapest generic equivalent would only push another 3 percent below the poverty line.

In Nigeria, amoxicillin is already unaffordable to the 56 percent of the population who live on less than $1.25 per day. A further 23 percent of Nigerians would be pushed into poverty by having to buy branded version Amoxil, made by GlaxoSmithKline, while only another 12 percent would be impoverished by buying the cheapest generic version.