Caffeine causes dehydration. It acts as a mild diuretic, but the fluid you consume in most caffeinated beverages tends to offset the effects of fluid loss when you urinate.
Caffeine helps you sober up. You might think you’re OK to get behind the wheel if you’ve had a coffee after a few alcoholic bevvies, but your reaction time and judgement will still be impaired.
Caffeine is addictive. Regular consumption does cause mild physical dependence, and if you suddenly stop taking caffeine you may have withdrawal symptoms – including headache, fatigue and irritability – for a day or more. But caffeine dependence isn’t generally considered a serious addiction.