How Long Does 100 Mg of Caffeine Last?

The noticeable effects of 100 mg of caffeine may last about 4–6 hours, though some caffeine can remain in your body for a day or longer. Your metabolism, medications, pregnancy, smoking, liver function and sensitivity can change both timelines.

Use the caffeine detox calculator to estimate what may remain, or read about how long caffeine stays in your system for the broader picture.

How much of 100 mg of caffeine remains over time?

Caffeine has a half-life of roughly 3–7 hours in many adults. A half-life is the time it takes for the amount in the body to fall by half; it is not the time when caffeine suddenly stops working.

The table uses an illustrative 5-hour half-life and assumes no more caffeine is consumed. It treats the full 100 mg dose as if it were absorbed at time zero, although absorption actually takes time. The amounts are estimates, not measurements of your blood caffeine level.

To make the calculation, halve the previous amount for each 5-hour period. For example, 50 mg becomes 25 mg after another 5 hours. With a different half-life, the estimates would change substantially.

Feeling back to normal does not mean all the caffeine is gone. Equally, a small amount remaining does not tell you exactly how alert you will feel or whether you will sleep well.

What changes how long caffeine lasts?

Metabolism and sensitivity

People process and respond to caffeine differently. Genetics can affect both how quickly the liver breaks down caffeine and how strongly a person feels its effects. A 2018 review of individual differences in caffeine metabolism describes these differences; it does not provide a precise timeline for any one person.

Pregnancy, liver function and smoking

Pregnancy and liver disease can slow caffeine clearance, so caffeine may remain longer than the table suggests. Smoking can increase the rate of caffeine metabolism; stopping smoking can change that rate again.

These factors do not make the table a personal prediction. If more than one applies to you, the combined effect is especially hard to estimate without medical advice.

Medications and additional caffeine

Some medications affect how quickly caffeine is broken down. For example, fluvoxamine and ciprofloxacin can slow its clearance. Ask a pharmacist or clinician about possible interactions with your own medicines; do not stop a prescribed medication to change how caffeine affects you.

A later dose adds to whatever remains from the first one. Check the labels of any caffeine-containing products you use, since a calculation based on 100 mg alone will not account for other sources.

Sleep and personal response

Caffeine can interfere with sleep even when its energizing effect feels less obvious. A review of caffeine and sleep describes its potential to disrupt subsequent sleep. If you notice a problem, consider having caffeine earlier or reducing it rather than relying on a clearance estimate as a bedtime guarantee.

Can the calculator predict when caffeine will wear off?

The calculator linked above can help you estimate caffeine remaining after a dose. Use 100 mg as the starting amount and consider the time since you took it. If the tool offers a half-life setting, remember that the default may not match your metabolism.

The result is an estimate of caffeine remaining, not a test of your alertness or fitness to drive. It also cannot establish when you will sleep normally. The table above offers a way to check the calculation using a 5-hour half-life.

Who should be careful with caffeine?

Pregnant or breastfeeding people, children and adolescents, and people with heart rhythm problems, anxiety disorders or liver disease should ask a healthcare professional for individual advice. A review of caffeine safety identifies these and other groups as potentially more vulnerable to caffeine's effects.

Contact a clinician if caffeine repeatedly causes troublesome palpitations, anxiety or sleep problems. Seek urgent medical help for chest pain, fainting, difficulty breathing, seizures or a sustained rapid or irregular heartbeat. If you suspect a dangerous caffeine ingestion, contact poison control or emergency services rather than waiting for it to wear off.