Mefloquine, the anti malarial drug, is a major cause of post malarial neurological syndrome (PMNS), where the patient can suffer convulsions and tremors.
Researchers who made the discovery are urging doctors to prescribe a safer alternative.
"Mefloquine should not be used after treatment of severe malaria," they recommend.
In the study, 4.4 per cent of patients who were given mefloquine went on to suffer PMNS, compared with just 0.5 per cent of those given quinine. Symptoms included an acute, confused state or psychosis, and general convulsions.
Doctors from two centres in Vietnam studied 18,124 malaria patients, 22 of whom went on to develop PMNS, usually after suffering severe malaria.
!AThe Lancet, 1996; 348: 917-21.