National Coffee Day: Do You Know What's in Your Joe?

(CBS) It's National Coffee Day and Americans are scrambling to grab their free cup of coffee (more on that in a minute). But not all coffee is created equally and before you double down on a double latte, it's good to know the health effects, good and bad, of coffee's active ingredient - caffeine.

According to the Mayo Clinic, the amount of caffeine in different brands of coffee differs dramatically. For example, a 16-ounce Dunkin Donuts brewed coffee can have anywhere from two to four times as much caffeine as a Starbucks espresso. Clearly, the bigger cup makes a big difference.

Here's a short list of caffeine content:

• Dunkin' Donuts, brewed, 16 oz (480 mL) 143-206
• Generic brewed, 8 oz (240 mL) 95-200
• Generic brewed, decaffeinated, 8 oz (240 mL) 2-12
• Generic instant, 8 oz (240 mL) 27-173
• Generic instant, decaffeinated, 8 oz (240 mL) 2-12
• Starbucks Espresso, 1 oz (30 mL) 58-75
• Starbucks Vanilla Latte, 16 oz (480 mL) 150

Whether you go big or small, it's good to know what effect caffeine is having on your body.

For those with anxiety, insomnia and a bad belly, caffeine isn't going to help. In fact, the Mayo Clinic warns that heavy caffeine consumption - more than six cups a day - can cause all kinds of ugly symptoms such as irregular heartbeat, muscle tremors, headaches, and of course inability to sleep and irritability.

There are also certain medications that don't mix well with caffeine including common antibiotics like ciprofloxacin (Cipro) and norfloxacin (Noroxin). Theophylline, a drug that helps with bronchial problems, also doesn't mix well, neither does the banned herbal supplement ephedra. Ask your doctor for a full list.

But the news on caffeine isn't all bad. In fact, much of it is very good.

Researchers have debunked the long held belief that caffeine contributes to heart attacks and cancer. It doesn't.

And according to the Mayo Clinic, coffee may actually help a laundry list of disorders including type 2 diabetes, Parkinson's, asthma and even gallstones.

So as long as your hands aren't shaking, drink up America. Enjoy your National Coffee Day. As for those free coffees? Here's a list.

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