Caffeine is a central nervous system stimulant. It can temporarily make you feel more awake and energetic, but it can also give you the jitters. Withdrawal or overdose can cause a range of problems.
People with voice disorders, singers, and other voice professionals are often advised against using caffeine. However, until recently, this recommendation was based only on hearsay. Now developing research seems to indicate that caffeine may actually harm voice quality. But further study is necessary to confirm these early findings