Caffeine

What it is

Caffeine is a plant-derived stimulant of the methylxanthine group. It blocks adenosine receptors in the brain, holding off the signal that normally accumulates during waking hours and is felt as tiredness, and it raises circulating adrenaline. It is taken for alertness and concentration, for a sense of energy, and before exercise for physical performance.

A usual adult range, not a national label — the dose and the timing differ by country and by product.

Used for

• Physical performance
• Energy
• Cognition

Approved uses differ by country — some of these may be off-label or unapproved where you live.

What to know while taking it

It stays in the blood long enough that an afternoon serving is still active at bedtime, which is why it belongs in the morning and why disturbed sleep is the commonest complaint. It can push the heart rate up and shift blood pressure, and regular use builds tolerance and physical dependence, so an abrupt stop brings headache, fatigue and irritability for several days. Ciprofloxacin blocks the liver enzyme that clears caffeine, so the usual amount then behaves like a far larger one. The stated daily maximum counts every source together: coffee, tea, cola, energy drinks and pre-workout powders.

When to see a doctor

A racing or irregular heartbeat, chest pain, persistent tremor, or insomnia that does not settle once intake is reduced are reasons to see a doctor. So is any of this appearing during a course of ciprofloxacin.

Worth knowing

• Can disturb sleep
• Can raise the heart rate
• Can raise blood pressure
• Can cause dependence
• Do not exceed the daily maximum