By Jon Johnson

Caffeine is a familiar drug that stimulates the nervous system. When it enters the body, caffeine raises the heart rate and blood pressure, increasing energy levels and improving the mood.

Caffeine acts quickly, and many people notice the effects within minutes. They last until the body fully metabolizes the drug. This duration depends on several factors.

Each person will feel the effects differently, and some may last longer than others. People who are pregnant or have difficulty sleeping should be careful about timing their caffeine intake and may want to avoid it altogether.

How long does it take to metabolize caffeine?

Caffeine has a half-life of about 5 hours.

Someone who consumes 40 milligrams (mg) of caffeine will have 20 mg remaining in their system after 5 hours.

When do effects peak?

Levels of caffeine peak in the blood within about 15–45 minutes of consumption.

They are then quickly metabolized by the liver.

Most people notice the strongest effects during this time, and many report feeling jittery, needing to urinate, and having sudden bursts of energy. These symptoms tend to go away as the caffeine starts to break down.

Can a person build up a tolerance?

As the body becomes resistant to the drug, people who regularly consume caffeine may barely notice its effects.

However, for someone who is very sensitive to caffeine, effects may persist for hours or until the next day.

How long do effects last?

There is no set time limit. The duration of the drug's effects depend upon the dosage and on personal factors, including age, body weight, and how sensitive a person is to caffeine.

Foods and drinks that contain caffeine

Caffeine is usually found in beverages, including:

-coffee and drinks such as espressos, lattes, and cappuccinos

-black, green, and white tea

-yerba maté

-many soft drinks

-energy drinks

Even decaffeinated coffee contains some caffeine, and people who are very sensitive to caffeine should avoid it.

Caffeine can also be found in foods, such as:

-chocolate and products containing it, such as hot cocoa

-coffee or mocha ice creams

-guarana seeds and beverages

-some protein and energy bars

-pre-workout drinks and powders

Caffeine is also a common ingredient in weight loss pills and over-the-counter headache medicines, including Excedrin.