A treatment for malaria has saved millions of lives and led to this year’s Nobel Prize for medicine, awarded earlier this week. But we may not be able to rely on the drug for much longer: The parasite that causes malaria is becoming resistant to it.

When malaria patients are treated, there’s always a chance that some of the parasites in their bloodstream carry a mutation that protects them against the drug. After the susceptible parasites die, the resistant ones remain. Over time, more and more cases are caused by the drug-resistant parasites that have survived previous treatments. Malaria has already become resistant to older drugs.

The prize-winning treatment, called artemisinin, was isolated in the 1970s by Tu Youyou, a Chinese medical scientist who shared her Nobel with two other scientists who worked on a different project. Artemisinin used to be administered on its own, but, in order to lower the chance that drug-resistant strains will survive, now it’s never supposed to be prescribed in isolation. Pairing artemisinin with another drug typically means a parasite must be resistant to both drugs to survive.

The World Health Organization recommends that artemisinin-based drug cocktails be the first-line treatment for most malaria cases. Parasitology expert Colin Sutherland of the London School of Hygiene and Tropical Medicine estimates that these measures could keep artemisinin useful for five to 10 more years.

In order to put off resistance for as long as possible, the WHO has recommended discontinuing drugs in any country where treatment fails at least 10 percent of the time. The WHO monitors resistance worldwide to catch and retire weakening drugs before resistance becomes widespread. Artemether-lumefantrine and artesunate-amodiaquine are the artemisinin combinations for which the WHO has the most extensive data on resistance.

Cambodia is the only country where one of these two combination drugs (artemether-lumefantrine) already has a median treatment failure rate above 10 percent. Ten of the remaining 48 countries with artemether-lumefantrine resistance studies have had at least one study that found a treatment failure rate of greater than 10 percent. In four of the 30 countries where artesunate-amodiaquine was tested, the failure rate was above 10 percent in at least one study.

Increased resistance would be devastating. Half the world’s population is at risk of getting malaria, so any decrease in drugs’ efficacy could result in many more deaths. In 2013, the WHO estimates, there were 198 million cases of malaria worldwide and 584,000 deaths as a result of the disease. The vast majority of those who died (78 percent) were children younger than 5.

Artemisinin and Tu’s dogged research deserve a great deal of credit for the overall death rate not being much higher. The WHO’s recommendations should allow the world to benefit from her work for as long as possible. But, as the Nobel committee celebrates her, the world is desperate for the next Tu.