Studies in Pakistan showed that diclofenac caused acute kidney failure in vultures when they ate the carcasses of animals that had recently been treated with it (see below at Ecological problems). Species and individual humans that are drug sensitive are initially assumed to lack genes expressing specific drug detoxification enzymes. Because elderly humans have reduced expression levels of all enzymes, elderly human metabolisms may gradually approach those of vultures, thus unexpectedly becoming more diclofenac intolerant.