Caffeine

Yes. Caffeine is one of the few aids with solid evidence: it improves endurance and makes effort feel easier.

• Effective dose: about 3 mg per kg of body weight (around 200 mg for 70 kg); more adds little but more side effects.
• Take it 30 to 60 minutes before; in long races, some can come later from caffeinated gels.
• A cup of filter coffee holds about 80 to 120 mg, an espresso about 60 to 80 mg.
• Side effects: jitters, upset stomach, faster heart rate, poor sleep if taken late.
• Test it in training, never for the first time on race day.

Ask your doctor first if you have a heart condition or high blood pressure, or are pregnant.