Caffeine is the prime suspect in the ocular keratitis, dryness of the eyes, that sidelined big-league slugger Josh Hamilton recently.

Hamilton had described the lack of focus he was experiencing as being on 'an extreme sugar high.' The problem was instead ultimately traced to his caffeine consumption.

"Drinking caffeine -- coffee in the morning, coffee midday, energy drink before the game, chocolate after the game -- all these things were compounding and making it worse and worse," said Hamilton.

But some results show caffeine can help people avoid everything from heart disease to Parkinson's disease.

The blog Mother Nature Network details several studies:

•  Scientists at Harvard University examined data from 67,470 middle-aged women who were followed for about 26 years. When compared to women who drank little or no coffee, the ones who drank four or more cups per day had a 25 percent lower risk of developing endometrial cancer. Those who drank two or three cups per day had a 7 percent lower risk.
  
  
• According to a 2009 meta-analysis, at least fourteen out of eighteen cohort studies revealed a substantially lower risk of type 2 diabetes mellitus with frequent coffee intake, and that the risk of type 2 diabetes goes down with each cup of coffee consumed daily.
  
  
• Studies have demonstrated that those who use caffeine are less likely to develop Parkinson's disease. The latest study examining the connection found that caffeine can also help with movement symptoms for people who already have the disease.
  
  
• The New England Journal of Medicine published a study that found that increased coffee consumption is linked to longer a longer life. Coffee drinkers didn't die as early from heart or respiratory disease, stroke, diabetes, injuries, accidents or infections, and in fact, they decreased their risk of death from these factors by 10 to 16 percent.

So is that morning cup of coffee, that so many of us are addicted to, good or bad?

"There's not a lot of good research on it just yet," said St. John Medical Center Dr. Theron Bliss.

He said there's not much financial incentive for drug companies to study something that is so readily available to the public in hundreds of forms.

The only good consensus he says is that caffeine can boost athletic performance, which is why he says it's monitored in Olympic and college athletes.

The downside he says is that it elevates blood pressure.

Other than those two things, he says all the claims, both good and bad, are largely unclear and unproven.