Unless you have heart or kidney problems, the danger from 1 or 2 caffeine tablets is probably minimal, (as long as you don't abuse them...)
Caffeine is a diuretic. It causes your body to lose water through your kidneys. In other words, if you take a lot, you tend to just pee it out. It's also broken down in the liver in about 2-4 hours. However, I would advise against taking more than 400 mg at a time. most caffeine tablets are either 100 or 200 mg. An average cup of coffee (7 fl. oz.) has about 80-120 mg of caffeine.
Caffeine is a stimulant. Like all stimulants, repeated use leads to resistance and physical dependence. If you take it on a regular basis, then you become more and more immune to it's effects, and you have to keep taking progressively larger doses. This can become a vicious cycle that takes a toll on your health and well-being.
You say that you "get tired a lot and energy drinks aren't working and neither is coffee....."
This would be a good reason talk to a doctor, I think....
Chronic fatigue can be a sign of a more serious problem, such as sleep apnea, or diabetes for example.
Note: The first thing that comes to my mind, is that some of the "tiredness" is actually a symptom of caffeine withdrawal.
Also, caffeine doesn't *actually* give you more energy; it just blocks fatigue signals from reaching your brain. Your body may be trying to tell you something, but by taking all the caffeine you're not able to listen. At the end of the day, taking large amounts of caffeine will make you *more* tired, not less, because it encourages you to overexert yourself.
1 gram = 1,000 milligrams.
10 grams = 10,000 mg!
Net lethal dose can be as low as 2 grams! This is not a safe amount of caffeine to take!