When you drink 1 to 2 cups of coffee a day, it acts as a mild stimulant. The caffeine boosts your heart rate and blood pressure. This makes you feel more alert and better able to concentrate. Caffeine's effects start about 15 minutes after you drink it. The effects last up to several hours. Drinking more caffeine – more than 3 cups of coffee a day – can cause health problems. You may have anxiety, insomnia, headaches, stomach irritation, and an irregular heartbeat. These problems can also happen at lower amounts of caffeine if you are sensitive to it. People who regularly drink or eat caffeine may have a "withdrawal" 12 to 24 hours after they stop using it. The main symptom is headache, but other symptoms can include anxiety, fatigue, and drowsiness. These symptoms usually ease within a day or two.

That's about equal to two 12-ounce cups of brewed coffee. But heavy caffeine users may consume nearly 500 mg a day. Coffee and tea make up about 20% of caffeine intake for U.S. adults. Soft drinks make up about 5% of intake. For children and teens, tea and soft drinks provide the most caffeine in their diets. The average child takes in about 21 mg of caffeine a day.

Studies so far have not found a link between drinking only 1 to 2 cups of coffee a day and illnesses such as cancer or heart disease. Studies also have not found that caffeine affects fibrocystic breast disease. Research has found that drinking 3 or more cups of coffee a day can lead to spinal bone loss and thinning of other bones. This is especially true in older women. Caffeine also can interfere with bone development in children. This is because caffeine makes it harder for the body to absorb calcium. Another study found that moderate caffeine consumption may help prevent type 2 diabetes.

Research has found that polyphenols in green tea may offer protection against certain types of cancer.

A cup of coffee made in an automatic drip maker contains 137 mg of caffeine. A cup made in an automatic percolator has only 117 mg. Ounce for ounce, coffee has much more caffeine than tea. A cup of black tea brewed for 5 minutes has 65 mg of caffeine. A cup of green tea has 31 mg. Soft drinks also have caffeine. Colas and Mountain Dew have 36 to 54 mg of caffeine per 12-ounce can. Cocoa, on the other hand, has only 5 mg of caffeine in 6 ounces.

Check the product label to find out the amount of caffeine in OTC products.

Instant coffee generally has less caffeine than brewed coffee. If you brew your tea for about a minute, instead of the usual 3 minutes, you can cut the amount of caffeine in half. Avoiding caffeine after noon can help if you have trouble sleeping at night because of too much caffeine. Here are other ideas:

• Brew half caffeinated/half decaffeinated coffee.
• Switch to decaffeinated or herbal teas.
• Don't buy soft drinks that contain caffeine. Most clear soft drinks are caffeine-free, as are flavors like orange and root beer, but check the label.

Also keep in mind that some medicines have caffeine. Talk with your healthcare provider or pharmacist if you have questions about caffeine and medicine.