As my brother neared the end of a smooth, swift recovery from open-heart surgery to bypass an 80-percent blockage in his heart’s most important artery recently, he reverted to a longstanding habit of downing many cups a day of strong coffee. I objected, but he insisted that the caffeine doesn’t affect him, meaning it doesn’t disturb his sleep.

But when I noticed how easily he became upset or angered by minor irritations many times a day, I decided to look into what is known about the bodily effects of so much caffeine and whether it might contribute to the harmful effects of stress on the heart.

Caffeine is by far America’s leading nonprescription drug, regularly consumed by some 90 million adults each day in coffee, tea, soft and energy drinks and some prescription and over-the-counter medications.

Amounts can range widely. An 8-ounce cup of brewed coffee may contain 95 to 165 milligrams of caffeine, while a cup of instant coffee delivers 63 milligrams. One ounce of espresso has 47 to 64 milligrams; 8 ounces of brewed black tea, 25 to 48 milligrams; 8 ounces of brewed green tea, 25 to 29 milligrams; 8 ounces of cola, 24 to 46 milligrams, whereas a 12-ounce energy drink can contain as much as 300 milligrams. Much smaller amounts of caffeine can be found in chocolate, decaffeinated coffee, some candies and even some foods like waffles.