"People often don't understand the potential risk of these beverages," University of Florida's director of forensic toxicology, Bruce A. Goldberger, told Chemical and Engineering News. "Caffeine is a stimulant and, when consumed at high enough levels, can have negative effects."
"Whereas low-dose caffeine effects are wakefulness, a little bit of arousal, and slight euphoria," Bertil B. Fredholm, emeritus professor of pharmacology at Sweden's Karolinska Institute, said in the report, "high-dose effects are anxiety, irritation, and general mental discomfort -- a completely different kettle of fish."
Yet scientists say that tolerance levels for caffeine can vary widely between individuals. Drug use can also affect caffeine tolerance. Women who take contraceptive pills break down caffeine more slowly, while smokers tend to process the stimulant at a faster pace than non-smokers.
Caffeine recommendations and levels in popular drinks, as per Chemical and Engineering News:
Recommended daily limit for a healthy adult: 400 mg (Source: Health Canada)
Grande (16 oz./ 473 mL) cup of Starbucks coffee: 330 mg
2 oz./59 mL show of 5-Hour Energy: 215 mg
Recommended daily limit for a pregnant woman: 200 mg (Source: Food Standards Agency)
8 oz./236 mL cup of coffee (brewed): 133 mg
20 oz./591 mL bottle of Pepsi Max: 115 mg
8 oz./236 mL cup of coffee (instant): 93 mg
8 oz./236 mL can of Monster Energy : 92 mg
20 oz./591 mL bottle of Mountain Dew: 90 mg
8 oz./236 mL can of Red Bull Enegy Drink: 83 mg
2 oz./59 mL cup of espresso: 80 mg
Recommended daily limit for a healthy 10-year-old: 80 mg (Source: Food Standards Agency)
20 oz./591 mL bottle of Coca-Cola: 58 mg
8 oz./236 mL cup of black tea (brewed): 55 mg
8 oz./236 mL cup of hot chocolate: 9 mg
8 oz./236 mL cup of coffee (decaf): 5 mg