Moderate negative effects of caffeine include:

    * Headaches
    * Irritability
    * Increase in breast tenderness
    * Restlessness
    * Mild insomnia
    * Increased heart rate
    * Increased blood sugar
    * Difficulty concentrating
    * Mild stomach upset

Severe negative effects of caffeine include:

    * Panic attacks
    * Irregular heartbeat
    * Sleep deprivation
    * Continuous stomach problems
    * Prolonged depression
    * Sleep disorders

The negative effects of caffeine are well documented, but not all research agrees on the results of caffeine. Some is specifically caffeine negative, citing numerous negative effects of caffeine. Other studies take a more positive approach and cite both benefits and possible problems with caffeine consumption. Most studies correlate the negative effects of caffeine with high caffeine intake, and with certain physical conditions or groups of people who tend to have more problems with caffeine.

A small amount of caffeine intake by a completely healthy person may have minimal effect. Caffeine is a stimulant and it will elevate heart rate, increase blood flow, and raise body temperature. It increases blood sugar levels and act as a diuretic, meaning you urinate more frequently and reduce water intake.

One of the negative effects of caffeine is that small amounts of caffeine taken daily, for example a cup of coffee a day, can create physical dependence on caffeine. If you fail to drink your daily dose of caffeine, you can end up with headaches, excessive sleepiness, a feeling like you can’t function or flu like symptoms. Withdrawal from caffeine can take anywhere from a few days to a few weeks depending upon how much caffeine you customarily consume.

When caffeine intake is above 300 milligrams a day, about three cups of coffee or three to four sodas, the caffeine may at first make you feel energized. Yet one of the negative effects of caffeine is its ability to disrupt sleep cycles, causing less deep sleep, particularly when you have caffeinated beverages within a few hours of bedtime. So while you may be using coffee or other caffeine laced beverages to “get you started” in the morning, you likely need this extra boost because you are getting inadequate sleep.

In 2006, studies on night shift workers found that those who consume caffeine at night are most prone to this effect. Since night shift work already disrupts normal sleeping patterns, caffeine intake may create more serious issues. Those who drink caffeinated beverages and work the night shift have a far greater likelihood of getting insufficient deep sleep during the day. This can result in a continued cycle of exhaustion and caffeine boosts, a greater dependence on caffeine, and more caffeine consumed, which only exacerbates the problem.

Certain people are also particularly caffeine sensitive and more likely to experience negative effects of caffeine. For example, some people will feel “buzzed” or hyped up from one cup of coffee. Lower body weight tends to translate to higher likelihood of experiencing the negative effects of caffeine.

People with panic disorder or generalized anxiety disorder are much more prone to reacting badly from increased heart rate. Caffeine, even in small doses, can create panic attacks and interfere with medications taken to calm the system. Caffeine does reduce dopamine, a chemical produced by the brain that affects the brain’s pleasure centers. This can create more depression and anxiety.

For people with high blood pressure or high blood sugar levels (diabetes) the negative effects of caffeine should be understood. Caffeine can raise both these levels. Any type of heart problem can be affected by caffeine. The negative effect of caffeine in increasing heart rate can create problems for people with heart conditions, and in high doses, caffeine can induce irregular heartbeats in healthy people.

People with stomach conditions like irritable bowel syndrome, Crohn’s disease, ulcers, and gastro-intestinal reflux, may all suffer from the negative effects of caffeine. Caffeine increases acidic response in the stomach which may lead to excessive bowel movements and is associated with stomach discomfort.

Though there are some positive effects of caffeine, the negative effects of caffeine should be understood. Be aware if you are in a high-risk group for severe negative effects. If you have mood disorders, high blood pressure, diabetes, heart disease, kidney disease, chronic intestinal issues, work the night shift or are pregnant, you should minimize caffeine intake.

Taken from  http://www.wisegeek.com/what-are-the-negative-effects-of-caffeine.htm
Written by Tricia Ellis-Christensen