GlaxoSmithKline PLC announced Friday that it would halt development of its malaria drug Dacart and would stop selling another drug Lapdap because both appear to lead to anemia.

Malaria is spread by mosquitos and it generally appears in tropical and subtropical regions, including parts of Asia, and Africa. Malaria kills more than one million people each year, mostly in the developing world, according to the World Health Organization.

According to tests conducted by GSK and its Partner Medicines for Malaria Venture, both drugs lowered hemoglobin concentration in patients with hereditary enzyme deficiency disorders, which affects 10 to 25 percent of the sub-Saharan population in Africa. A deficiency of hemoglobin can lead to anemia, which reduces the amount of oxygen available to the body and causes fatigue.

The tests showed that patients taking Dacart experienced a bigger drop in hemoglobin levels that those taking Novartis AG’s Coartem, GSK said in a statement according to the Wall Street Journal. Dacart combines Lapdap with a form of artemisinin, a natural substance derived from a Chinese plant.

Also, patients taking Lapdap had the same results as patients taking Dacart, which prompted GSK to recall it from pharmacies in Kenya, the only country where it is sold, the company added.

GSK’s announcement reduces the number of available malaria treatments on the market. Health experts encourage companies to put as many drugs on the market as possible in an effort to increase competition and reduce prices.