The World Health Organization (WHO) has asked nearly 20 pharmaceutical firms to stop selling "single-drug" artemisinin malaria medicines to prevent malaria parasites from developing resistance to the treatment.

The use of single-drug artemisinin treatment -- or monotherapy -- hastens development of resistance by weakening but not killing the parasite, a top WHO official said in a statement last week.

"It is critical that artemisinins be used correctly," said Lee Jong-wook, the WHO's director-general. "We request pharmaceutical companies to immediately stop marketing single-drug artemisinin tablets and instead market artemisinin combination therapies only."

The WHO recently released new treatment guidelines to provide countries with clear and evidence-based direction on the best treatment options for malaria. In Thailand, for example, sulfadoxine-pyrimethanime was almost 100 percent effective in curing malaria when introduced in 1977. But within five years, it was curing only 10 percent of cases due to drug resistance.

WHO also urged the research community and pharmaceutical industry to rapidly invest in the design of the next generation of antimalarial drugs. By creating artemisinin combination therapies (ACTs) with multiple-drug combinations and transmission blocking components, resistance can be prevented, the group said.