When Howard D. Schultz in 1985 founded the company that would become the wildly successful Starbucks chain, no financial adviser had to tell him that coffee was America’s leading beverage and caffeine its most widely used drug. The millions of customers who flock to Starbucks to order a double espresso, latte or coffee grande attest daily to his assessment of American passions.

Although the company might have overestimated consumer willingness to spend up to $4 for a cup of coffee  it recently announced that it would close hundreds of underperforming stores  scores of imitators that now sell coffee, tea and other products laced with caffeine reflect a society determined to run hard on as little sleep as possible.

But as with any product used to excess, consumers often wonder about the health consequences. And researchers readily oblige. Hardly a month goes by without a report that hails coffee, tea or caffeine as healthful or damns them as potential killers.

Can all these often contradictory reports be right? Yes. Coffee and tea, after all, are complex mixtures of chemicals, several of which may independently affect health.