Caffeine is a central nervous system pick-me-up, and is used in two ways:  recreationally and medically to re-establish mental alertness when extraordinary weakness or lethargy occurs.  Caffeine should not be a substitute for  sleep, and should be used only once in a while to become more alert.

Caffeine can be taken together with medicines to enhance their efficacy, such as with ergotamine in the healing of migraines, or with specific pain killers such as aspirin or acetaminophen. Caffeine may also be used to defeat the drowsiness caused by antihistamines. Breathing problems in premature babies is now and then treated with citrated caffeine, which is accessible only by prescription.  While relatively not dangerous for humans, caffeine is much more toxic to some other animals such as dogs due to a much poorer capacity to metabolize these substances.

There is some evidence that the same may be true for newborns. Therefore, some doctors advise that women largely stop the use of caffeine while pregnant and until they stop breastfeeding.