CNN did a story on caffeine overdosing and I thought it was interesting because they had some parameters from The National Institutes of Health.

According to the guidelines the NIH put forth, I think a lot of people are in danger of OD-ing. The doctors recommend taking in no more than 200-300 milligrams of caffeine per day; according to the Mayo Clinic's website, there are 300 milligrams of caffeine in one 16 ounce cup of brewed Pike's Peak from Starbucks. Now if you only have one of those a day, you should be fine, but a lot of people make multiple trips. Sixteen ounces of coffee at McDonald's has only 100 mg of caffeine, and the stuff you brew at home has between 95 and 200 milligrams. How do you know if you are overdosing on caffeine?

According to the CNN story, breathing trouble, convulsions, fever, irregular heartbeat, dizziness, hallucinations, even seizures are possible. That's if you take in more than 500-600 milligrams and enter caffeine poisoning. Given the amount of caffeine in those coffees as well as in energy drinks (~240 mg per can), it seems like those dangerous levels would be easily achieved.