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QUINCY, ILL. -- You've probably heard about that 2:30 feeling...

You're tired and can't think...

You reach for a cup of coffee or grab an energy drink.

But are these energy drinks and shots healthy for you?

The most active ingredient in energy drinks and energy shots is caffeine.

"Caffeine is a stimulant. It acts as a diuretic as well. It can raise your heart rate, can increase your blood pressure, cause palpatations. It might cause a little bit of GI distress or nausea. You may think you're hydrating but you're actually not-- you can lose fluids. You have to keep up on your water if you're going to use caffeine," said Gina Pontius, M.D. with Sothern Illinois University School of Medicine.

5 hour energy shots say they have as much caffeine in them as one cup of coffee -- 180 milligrams.

NOS Power Shots have 125 milligrams and RockStar's Energy shot came in the highest with 200 milligrams. Doctors suggest that anywhere from 200 to 300 milligrams of caffeine a day is moderate.

"Everything in moderation. Probably a cup or two of coffee a day isn't a big deal but whenever you get up to 3 cups or a whole pot of coffee a day... That's probably too much coffee...Caffeine over time can be addicting and you start to kind of need it to get you going and then there's withdrawal effects too. You can feel nauseous, light headed... Just start to depend on it a little bit," said Pontius.

While caffeine and energy drinks might be alright in moderation ...

"It's not an appropriate substitute for adequate rest, exercise, proper nutrition, and appropriate hydration. Definitely there can be an underlying cause of fatigue. You could be masking that in some way with caffeine to some degree," said Pontius.

According to research company Mintel, energy drinks are a 4.9 billion dollar industry and growing every year.