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Caffeine is probably the world's most popular drug. Legal everywhere (I once had a cup of coffee at the Denny's across from Temple Square in Salt Lake City), caffeine is the pick me up most of us need every day. The ways caffeine affect us, however, differ from person to person. It's not all about how many milligrams of caffeine are in a cup of coffee or tea or in a glass of soft drink.


Caffeine doesn't enter the bloodstream instantly. It takes about 45 minutes for it to travel through the stomach to the small intestine into the bloodstream if the stomach is empty. 


If you have eaten something, however, the caffeine in the beverage will sit in your stomach until food is digested, all the nutrients emptied out of the stomach at the same time. The caffeine in coffee and a doughnut or coffee and cream or coffee and a candy bar, or coffee after a full meal, takes longer to "hit" the bloodstream. That's also true of other sources of caffeine, of course, including caffeine pills like No-Doze and Vivarin.

All other things being equal, caffeine has a half life in the bloodstream of 5 to 7 hours. That means that 50 percent of the caffeine in your cup of coffee or tea with breakfast is still in your bloodstream 5 to 7 hours later, and 25 percent is still in your bloodstream 10 to 14 hours later. However, the other substances you use can affect how long caffeine stays in your system. 

Nicotine causes your liver to clear caffeine out of your system 30 to 50 percent faster. Instead of feeling an energy boost for two or three hours, if you smoke, the effects of caffeine may last just an hour to 90 minutes.

In women, the use of oral contraceptives has the opposite effect on caffeine clearance. In women who are on the Pill, caffeine has a half life of 8 to 12 hours. In women who are pregnant, the added estrogen and pregesterone in their bodies gives caffeine a half life of about 15 to 16 hours. 

Men or women who take SSRI antidepressants clear caffeine very slowly. For them, 50 percent of caffeine from a single beverage may stay in their systems as long as 56 hours. This means that caffeine may build up in the bloodstreams of people who use drugs such as Luvox. 


In newborns, caffeine has a half life of up to 80 hours, although by the time a baby is six months old, he or she has developed enzymes that can process the substance.


What caffeine does for you also depends on how much you are giving your liver to detoxify. The liver transforms caffeine into theophylline, the same chemical that is also used as an asthma medication. It turns caffeine into theobromine, the chemical found in chocolate, which relaxes blood vessels and sets the mood for love. It turns most of caffeine into paraxanthine, which helps fat cells release fat stores as  glycerol and free fatty acids so the muscles can burn them.

However, all of these tranformations depend on an enzyme known as CYP1A2. Some foods and substances increase the activity of CYP1A2. If you eat cauliflower, or grilled meat, or broccoli, or Brussels sprouts, your body will use the enzyme to transform caffeine into one of these three (or a closely related) non-stimulant form. 


On the other hand, grapefruit juice, cumin, peppermint (candy or tea), and chamomile tea interfere with the enzyme, so the liver keeps caffeine in its stimulant form. 


Many medications have intense effects on how your body uses caffeine. If you take insulin, your body "burns up" caffeine fast. If you take Cipro or calcium channel blockers or certain medications for GERD (such as Omeprazole, also known as Prilosec), your body quickly turns caffeine into other chemicals. If you take a different medication for GERD, cimetidine (Tagamet), or if you take St. John's wort or you eat a lot of curry, your body hangs on to caffeine longer. Many of the medications for psychiatric disorders also keep caffeine in its stimulant form.


It's not just how much caffeine you get into your system. It's also everything else you ingest. A little more or a little less caffeine may or may not have any effect at all.

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