​Caffeine

Caffeine is very effective when it comes to increasing mental and physical performance. (1) It's cheap, widely available and you're unlikely to have any side effects.
Benefits of caffeine for  training

• The main benefit of caffeine is that it makes you mentally more alert, more energy and puts you in a better mental state for your training. This is especially true if you haven't had much sleep recently, or life has been busy and you don't feel like training.
• Studies show it may increase endurance and high-intensity activity.(2,3,4,5,6)

Caffeine sources

• Coffee
• Tea
• Energy drinks
• ProPlus caffeine tablets

I like to use ProPlus tablets. It's easier for me to make sure I get exactly as much caffeine as I want without giving me coffee breath. I try to avoid energy drinks.

Caffeine dosage
Generally, 200mg to 400mg before a workout is recommended, but it will be different for everyone. I'm very sensitive t caffeine and get shakey jittery if I have too much. I tend to only take 100mg if I use caffeine before a workout. If you've not been much of a caffeine drinker in the past, it might be best to start low and build up to see how your body reacts.
Caffeine is rapidly absorbed into the bloodstream, and blood levels peak after 90-100 minutes. Caffeine levels remain high for 3 to 4 hours, and then start to drop.

Side effects
Most of the side effects of caffeine can be avoided by using a sensible dose,  However, it may be unsuitable for some people. Caffeine is not recommended for people who take certain medications, as well as those with a heart condition or high blood pressure

• Increased heart rate.
• Anxiety.
• Dizziness.
• Insomnia or sleep disruption. If this happens, try to not have any caffeine. too late in the day 
• Irritability.
• Tremors.
• Stomach discomfort.

Side effects aren't that common, and most are caused by using too much and a lower dose won't cause any problems.