If you have been cutting back on smoking, or have quit, you might have noticed an increase in your caffeine use. The Substance Abuse and Mental Health Services Administration's blog has an article about this.
Here's a quote:
For example, a smoker who usually drank two cups of coffee in the morning with a cigarette will feel stronger caffeine effects from the same two cups after quitting smoking. This is because smoking causes the body to metabolize caffeine more quickly. So, when you quit smoking and continue to drink coffee or other caffeinated drinks, your metabolism rate slows and the caffeine level in your body rises. This may cause caffeine toxicity which can result in anxiety, difficulty concentrating, impatience, insomnia, and restlessness. These symptoms are often attributed to nicotine withdrawal.
Did you know there is are actual diagnoses for caffeine intoxication and withdrawal? In order to not have withdrawal symptoms (headaches, irritability or dysphoric mood, problems concentrating, flu like symptoms) cut back on caffeine gradually.
To read more about quitting smoking and caffeine, here is the link to the article.