Caffeine can make some people feel highly alert while others barely notice its effects, even after drinking several cups of coffee. The difference is partly explained by how quickly the body processes caffeine.

Caffeine is a widely used psychoactive substance found in coffee, tea, cola, energy drinks, chocolate and some supplements. It can increase alertness and concentration by blocking adenosine, a chemical that builds up in the brain and contributes to feelings of tiredness.

After caffeine enters the bloodstream, the liver gradually breaks it down before the body removes it. This process can take roughly six to 20 hours, depending on the individual.

One important factor is the CYP1A2 gene, which influences how quickly the liver metabolises caffeine. Genetic differences can therefore affect how long caffeine remains in the body and how strongly someone experiences its effects.

However, genetics are not the only factor. Smoking can increase the rate at which caffeine is broken down, potentially making its effects shorter. People who stop smoking may therefore find that their usual amount of coffee affects them more strongly.

Certain medicines can have the opposite effect. Some antibiotics, contraceptive pills and other prescription drugs may slow caffeine metabolism, allowing it to remain in the body for longer.

The amount consumed also matters. While caffeine may improve alertness, excessive intake can cause symptoms such as headaches, shakiness, a racing heart and sleep disruption.