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When (consumed) in small quantities like, for example when you have one cup of coffee or one soda, caffeine can cause your heart rate to increase, you urinate more which can cause dehydration, and your digestive system produces more acid.
In larger amounts, caffeine can cause you to have headaches, feel restless and nervous, be unable to sleep, and even, in very large quantities to have (hallucinations).[Don't try that at home!] When larger amounts of caffeine [over 600 mg per day] are used over long periods of time you can develop sleep problems, get depressed and have problems with your (digestive system).
According to a Medline article on the National Institutes of Health website, having caffeine in your diet is not of any benefit to your health but moderate consumption is also not considered harmful.
They say that having up to 3 eight ounce cups of coffee a day or 250 mg of caffeine is considered [quote] "average or moderate". 10 cups of coffee a day is considered excessive. Also, remember that the amount of caffeine per cup can vary greatly depending on the type of beans that are used and the strength of the (brew).
Most sodas with caffeine, unless they are specially enhanced like "Jolt" or something like that, have about 35 mg of caffeine per 8 ounces so you don't have to worry too much unless you are drinking several 2 liter bottles per day. Also, the effect of caffeine on you personally will depend on a number of factors like your weight, (general health), mood and personal sensitivity to caffeine.
You can see that caffeine can have both (positive and negative effects) on our health and well-being but the bottom line is that if you drink your coffee or sodas in moderation, you don't have to worry too much.
So, the next time you are wondering whether you should have that second cup of coffee to perk you up, relax. At least now you know what it is and isn't doing to you!