Myth: Coffee makes you urinate so it dehydrates you.
Fact: Although caffeine is a mild diuretic whatever dehydration it might have caused is mostly offset by the water in your coffee.
Coffee is made of water so one would think it wouldn't dehydrate any more than soda, tea or other drinks. This myth arose because after ingesting a diuretic, there is an increased need to urinate. This ultimately led many to to believe that the coffee was dehydrating them.
In truth, with regular use, a person's body develops a tolerance to the ingestion of the caffeine in the coffee. This further negates the minor diuretic effect that is caused by the coffee.