Caffeine sensitivity: why caffeine affects people differently

Close-up of a caffeine pouch

Caffeine sensitivity: why caffeine affects people differently

People really do react to caffeine differently. Caffeine is broken down in the liver, mainly by an enzyme called CYP1A2, and how fast that happens varies a lot from person to person: in adults, caffeine's half-life is about 2 to 8 hours (EFSA). At very high intakes caffeine can cause anxiety, sleeplessness, agitation, palpitations, diarrhoea and restlessness, and the Food Standards Agency says these effects can be stronger in people who are sensitive to caffeine. A caffeine allergy is not the same thing as sensitivity: if you think you are having a reaction, speak to your GP or pharmacist.

Key points
• Caffeine is broken down in the liver, mainly by the enzyme CYP1A2 (EFSA).
• Half-life in adults: about 2 to 8 hours. In pregnancy it rises progressively, up to 16 hours (EFSA).
• Oestrogens and progestogens slow CYP1A2, which links to pregnancy and the contraceptive pill (EFSA).
• Researchers report large differences between individuals in how the body handles caffeine.
• Caffeine sensitivity is not an allergy. If you suspect a reaction, speak to your GP or pharmacist.

Table of contents

What is caffeine sensitivity?

"Caffeine sensitivity" and "caffeine intolerance" are everyday terms for noticing caffeine's effects more strongly, or at lower amounts, than other people do. The British Heart Foundation (BHF) notes that your heart rate may rise slightly after caffeine, especially if you are sensitive to its effects (BHF). More: How much caffeine is too much? · How long does caffeine last? · Caffeine pouches in the UK

Why people react differently to caffeine

Factor What the source says Source
Liver enzyme Caffeine is broken down mainly by cytochrome P450 1A2 (CYP1A2) in the liver EFSA; Grzegorzewski et al.
Half-life in adults about 2 to 8 hours EFSA
Pregnancy half-life rises progressively during pregnancy, up to 16 hours EFSA
Hormones oestrogens and progestogens slow CYP1A2 activity EFSA
Contraceptive pill studied as a factor that changes how the body handles caffeine Grzegorzewski et al.
Individual variation large differences between people in how caffeine is processed Grzegorzewski et al.
Genes genes including CYP1A2 and ADORA2A have been linked to how much caffeine people consume Tan et al.

Sources: EFSA stakeholder presentation, March 2015; Grzegorzewski et al., Frontiers in Pharmacology, 2021; Tan et al., Journal of Translational Medicine, 2024.

The half-life is the time it takes for the amount of caffeine in your blood to fall by half. A longer half-life means caffeine stays in your body for longer, so the same cup can affect one person for longer than another. The genetics study looked at how much caffeine people consume, not at sensitivity itself. More: How long does caffeine last?

Caffeine sensitivity symptoms

The Food Standards Agency (FSA) and Food Standards Scotland (FSS) say that at very high levels caffeine can cause:

  • anxiety
  • sleeplessness
  • agitation
  • palpitations
  • diarrhoea
  • restlessness

These effects may be stronger in people who are sensitive to caffeine, or who have heart disease or high blood pressure (FSA and FSS, 2024). The BHF adds that heart rate may rise slightly after caffeine, especially in people sensitive to it.

Caffeine can also affect sleep. In its summary of EFSA's 2015 opinion, EUFIC notes that doses of around 100 mg may affect sleep in some people, particularly close to bedtime (EUFIC). If you are thinking of cutting back, see Caffeine withdrawal.

Caffeine allergy vs caffeine intolerance

A true allergy is different from caffeine sensitivity or intolerance. This guide does not cover allergies. If you think you are having a reaction to caffeine or to something in a drink, speak to your GP or pharmacist.

How much caffeine is in common drinks?

If caffeine affects you strongly, it helps to know how much you are having:

Drink Serving Caffeine
Decaffeinated coffee 200 ml about 4 mg
Decaffeinated tea 250 ml about 8 mg
Green tea 250 ml about 38 mg
Cola 355 ml about 37 mg
Black tea 250 ml about 55 mg
Espresso 60 ml about 80 mg
Energy drink 250 ml about 80 mg
Filter coffee 200 ml about 90 mg

Source: EUFIC.

To add up your day, try our caffeine calculator. More: How much caffeine is in decaf coffee?

What the official limits say

Group Guidance Source
Healthy adults, single dose up to 200 mg does not raise safety concerns EFSA
Healthy adults, per day up to 400 mg unlikely to cause adverse effects FSA and FSS; EFSA
Pregnancy, per day 200 mg FSA and FSS
Children 3 mg per kg of body weight (a conservative safety level) EFSA, via EUFIC

Sources: EFSA, 2015; FSA and FSS, 2024; EUFIC.

These limits are for healthy people. The FSA notes that effects may be more severe in people who are sensitive to caffeine. If you have a heart condition or high blood pressure, speak to your GP or pharmacist. More: Caffeine and blood pressure

FAQ

Why does caffeine affect me more than other people?
People break down caffeine at different speeds. It is processed mainly by the liver enzyme CYP1A2, and the half-life in adults ranges from about 2 to 8 hours (EFSA).

What are the symptoms of caffeine sensitivity?
The FSA lists anxiety, sleeplessness, agitation, palpitations, diarrhoea and restlessness at very high intakes, and says effects may be stronger in sensitive people.

Can you be allergic to caffeine?
A true allergy is different from sensitivity. If you think you are having a reaction, speak to your GP or pharmacist.

Does pregnancy or the pill change how I handle caffeine?
EFSA notes that oestrogens and progestogens slow the enzyme that breaks down caffeine, and that caffeine's half-life rises during pregnancy, up to 16 hours. More: Caffeine in pregnancy

Read next: How long does caffeine last? · How much caffeine is too much? · Caffeine withdrawal

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