Study after study has found recently that moderate amounts of caffeine are not harmful. So healthy adults need not deny themselves coffee, tea, or other products unless the caffeine interferes with sleep.
Sages and philosophers from various world traditions have counseled “Everything in moderation,” and, stated another way, “Nothing in excess.” These aphorisms apply to caffeine, too, because too much of this otherwise beneficial chemical can cause insomnia, nervousness, muscle tremors and stomach upset.
Excess caffeine can also cause irritability, headaches, restlessness, excessive urination, and fast heartbeat. These side effects come with what MayoClinic.org  calls heavy caffeine use of 500 to 600 milligrams per day.
How much caffeine is safe?
In the correct amount, caffeine can alleviate fatigue and make a healthy adult feel more awake, focused, and concentrated. And it has other benefits, including enhancing the effectiveness of pain relievers, reducing inflammation, and alleviating migraines  and other types of headaches.
The jury is out on whether caffeine causes extra heartbeat. One 2016 study advises that it doesn't cause heart anomalies. An article at Medscape.com  says:
Whether or not they have the jitters, most regular coffee drinkers can at least be assured the caffeine probably isn’t giving them extra heartbeats, at least according to an analysis based on a cohort from the prospective Cardiovascular Health Study (CHS).
The stimulatory effects of high caffeine intake may cause your heart to beat faster.
It may also lead to altered heartbeat rhythm, called atrial fibrillation, which has been reported in young people who consumed energy drinks containing extremely high doses of caffeine.
But just exactly how much caffeine is safe? About 300 to 400 milligrams of caffeine are right for an adult, about 100 mg for adolescents and none for children, says Mayo.
The U.S. Food and Drug Administration states in a consumer update :
For healthy adults, the FDA has cited 400 milligrams a day—that's about four or five cups of coffee—as an amount not generally associated with dangerous, negative effects. However, there is wide variation in both how sensitive people are to the effects of caffeine and how fast they metabolize it (break it down).
Certain conditions tend to make people more sensitive to caffeine’s effects, as can some medications. In addition, if you’re pregnant, trying to become pregnant, or breastfeeding, or are concerned about another condition or medication, we recommend talking to your health care provider about whether you need to limit caffeine consumption.
The FDA has not set a level for children, but the American Academy of Pediatrics discourages the consumption of caffeine and other stimulants by children and adolescents.
Side effects of excess caffeine
The side effects of too much caffeine can be quite unpleasant, including anxiety, insomnia, digestive problems, muscle breakdown, and formation of a habit, Healthline warns .
That article also high blood pressure, a rapid heart rate, fatigue, and an urgent need to urinate frequently can come with too much caffeine.
The article gives warnings and symptoms of all those conditions but starts off by stating:
Most types containcaffeine, a substance that may boost your mood, metabolism and mental and physical performance.
Studies have also shown that it’s safe for most people when consumed in low-to-moderate amounts.
To avoid these symptoms, as we said, take no more than 400 mg of caffeine per day.
How much caffeine do products contain?
It is hard to know just how much caffeine you are getting, though. The amounts of caffeine in various products vary widely, and estimates given by various agencies and websites also differ in the amount of caffeine in each product.
Caffeine amounts in coffee vary so much that Viter Energy blog published a blog posting  exploring that beverage and many other products.
Eight ounces of the dark elixir vary anywhere from 100 to 270 mg of caffeine, depending on the variety of beans it’s made from and the type of brewing process. Brewed coffee averages about 163 mg of caffeine in 8 ounces.
CaffeineInformer.com reports  that there are 400 mg of caffeine are in five shots of espresso, about 11 12-ounce colas, five 8-ounce Red Bulls, one Starbucks 20-ounce brewed coffee and two 5 Hour Energy Shots.
The USDA reports black tea has about 72 mg of caffeine in 12 ounces, Extra Strength Excedrin pills have 65 mg per tablet, and Midol Complete gelcaps have 60 mg. Also, sodas vary from about 35 to 64 mg per can. A 41 gram bar of Hershey’s Special Dark Chocolate bar has 18 mg, and nine Hershey’s Kisses have about 9 mg of caffeine.
A good way to get your caffeine, especially if you are in a situation where you need to limit fluid intake, is in Viter Energy Mints .
The mints have 40 mg of caffeine per mint, so you can enjoy several without getting too much. They also have invigorating B vitamins and natural mint flavor to freshen your breath.
Black tea has about 72 mg of tea in 12 ounces. The cocoa in the chocolate cake too has small amounts of caffeine. (Photo by Nicubunu/Wikimedia Commons)
Correct amounts vary by person
But there are other factors besides caffeine content when considering how much is safe for adults, including a person’s own metabolism. A certain amount of caffeine can be good for one person but wrong for another person. Some people are more sensitive to caffeine’s effects, and the effects vary by age, weight and other factors.
Mayo clinic says:
How you react to caffeine may be determined in part by how much caffeine you’re used to drinking. People who don’t regularly drink caffeine tend to be more sensitive to its negative effects. Other factors may include body mass, age, medication use and health conditions such as anxiety disorders.
Research also suggests that men may be more susceptible to the effects of caffeine than are women.
People who don’t habitually drink coffee or take caffeine in tea, soda, energy drinks , drugs, chocolate or other candy may be the most prone to restlessness and even sleep loss if they have some in the late afternoon or in the evening.
To function at their best, most adults need about seven to eight hours of sleep every night. Too much caffeine can cause sleep loss and eventual chronic sleep deprivation, a longer-term problem than losing one night of sleep.
Beware sleep deprivation
Sleep deprivation decreases alertness and performance, impairs memory and thinking processes, puts stress on relationships and can cause occupational injuries or automobile accidents.
And it can be a vicious cycle. People who are losing sleep often drink coffee to feel alert, which makes it harder to fall asleep and stay asleep at night.
Newsy answers the question of the day in a YouTube video.
If you lost sleep, caffeine can help
But the news about caffeine and sleep is not all bad: “Because caffeine is a stimulant, most people use it after waking up in the morning or to remain alert during the day,” says the National Sleep Foundation. “While it is important to note that caffeine cannot replace sleep, it can temporarily make us feel more alert by blocking sleep-inducing chemicals in the brain and increasing adrenaline production.”
If you stick to no more than 400 mg, or three to four 8-ounce servings of coffee or several Viter Energy Mints  per day, you should be fine, and your performance and alertness may be enhanced. Many medical experts advise people not to take caffeine later than mid or late afternoon so sleep is not disrupted.
Doctors sometimes say an alcoholic drink or two per day, one for women and one for men, can be healthful. But they say if you haven't started drinking yet, you are better off remaining alcohol-free.
Maybe the same is true for caffeine. It can be a healthful, invigorating substance, but once you start taking it, many people form a habit that can briefly (for a few days) lead to headaches and extreme fatigue if you quit.
Erectile dysfunction. In combination, those are two of the ugliest words known to man. But can caffeine help you get it up?
Science hasn't found the definitive answer to this question, but one study concluded that fewer men who consume caffeine have problems performing. The study said:
Caffeine intake reduced the odds of prevalent ED, especially an intake equivalent to approximately 2-3 daily cups of coffee (170-375 mg/day). This reduction was also observed among overweight/obese and hypertensive, but not among diabetic men. Yet, these associations are warranted to be investigated in prospective studies