The caffeine in coffee can effect people in different ways, particularly depending on your size, gender and metabolism.
Many people experience ‘jitters’ which to some people is exciting, and others is scary. It can happen when someone has consumed too much coffee, and sometimes when people are experiencing withdrawal symptoms. For some people it can cause anxiety and irritability.
Coffee jitters are essentially palpitations by a process called vasoconstriction. According to wikipedia, vasoconstriction is ‘the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, particularly the large arteries and small arterioles… Generalized vasoconstriction usually results in an increase in systemic blood pressure, but it may also occur in specific tissues causing a localized reduction in blood flow.”
Vasoconstriction can also contribute to erectile dysfunction because this inhibits blood flow to the penis.
It generally effects people who are not regular coffee drinkers because their caffeine tolerance goes up.