http://en.wikipedia.org/wiki/Caffeine#Overuse
In large amounts, and especially over extended periods of time, caffeine can lead to a condition known as caffeinism.[67][68] Caffeinism usually combines caffeine dependency with a wide range of unpleasant physical and mental conditions including nervousness, irritability, anxiety, tremulousness, muscle twitching (hyperreflexia), insomnia, headaches, respiratory alkalosis, and heart palpitations.[69][70] Furthermore, because caffeine increases the production of stomach acid, high usage over time can lead to peptic ulcers, erosive esophagitis, and gastroesophageal reflux disease.[71]
One energy drink a day (which many would consider "moderation") can very well lead to these effects since the concentration of caffeine is so high.
You have to separate long term continuous consumption of high doses from occasional consumption. There is a big difference. You don't develop caffeinism just from occasionally drinking a can of energy drinks, which is again at most the same caffeine as in 2 cups of coffee.
What you can develop from just drinking a few energy drinks in one day if you are not used to caffeine is palpitations, possibly a temporary stomach upset, and difficulty falling asleep that night. If you experience that, you drinked too much... You will be fine the next day, just be aware of your tolerance and do go overboard.
Similarly, if you are a heavy habitual coffee drinker and notice you crave coffee a bit too much, feel tired when you are not drinking it, and have anxiety, you probably have gone overboard and should really cut down on it. Most habitual coffee drinkers do not fall into this category however, the same way that most people who eat food are not morbidly obese.
I personally don't like coffee so my intake of caffeine is usually mostly limited to chocolate, but again I think we all know how to tell these situations apart.
More people than you realize develop pre-cancer/cancerous conditions in the GI tract from the esophogus all the way to the colon from excessive caffeine intake. This is not that hard to do when you take in 150mg-350mg in a single can (ref: caffeine content of drinks. Moderation here is not so simple unless you are educated on fitness/nutrition.
I am not sure where you get the idea of caffeine causing digestive tract cancer. Evidence shows the opposite, that regular consumption of coffee (about 2 cups a day which is about 200-300mg of caffeine) -reduces- risk of colon cancer (see
http://jnci.oxfordjournals.org/cgi/content/abstract/97/4/282 for example).
Also, I have looked at several vitamin labels in the past -- no such warnings exist unless there is an abnormally high concentration of heavy metals (iron, aluminum, etc.) It is very hard to overdose on most vitamins. Comparing vitamin consumption to caffeine consumption here is a bit misplaced, in my opinion.
Vitamin labels do contain warnings, as they should.
Generally it's at least two warnings "WARNING: Accidental overdose of iron-containing products is a leading cause of fatal poisoning in children under 6..." and a general warning about the longer term possibility of overdose of vitamin A,D or E, ususally something like "do not take this product if taking other vitamin A supplements" etc.
Iron is not an abnormal nutrient, it's an -essential- one, but it just has a low dosage threshold for going from very healthy to toxic (while the threshold on things like water is very high).
Of course I agree it's very unlikely to overdose on vitamins, but that's because people mostly stick with directions instead of treating it as a food.
So again, caffeine has many health advantages, it has its place in training and health like many other supplements (vitamins, aminoacids, ginseng, green tea, etc).
It is -not- "bad for you", although it should be treated like vitamins and not go above healthy doses.
I would not recommend drinking energy drinks casually, and there are easy ways to tell if you went overboard even with just one, if you feel your heart is beating a bit too strongly or don't feel as good, you went overboard.
And at least some of the energy drinks out there are packaged more as recreational junk food you drink more for the weird colors, sugary taste and alertness, which has its place in life but it's not exactly what you would want to include in a normal healthy diet.
But otherwise again if you know what you are doing and planned carefully caffeine in the right healthy amounts and at the right strategic times helps your system burn more fat and spare muscles and has many other protective and beneficial advantages.
