Impair

impair [ im-PAIR ]
[ intransitive verb, transitive verb ]

MEANING :
1. (tr. v.) to cause to diminish in strength or value
2. (intr. v.) to become worse

EXAMPLE 1 :
The doctor said it was cataract that had impaired his vision.

EXAMPLE 2 :
The research, details of which were announced at the British Association's Festival of Science in Sheffield, UK, indicates that drastic dieting can impair memory and slow reaction times.