TL;DR If you notice unusual side effects after drinking coffee, then you may be experiencing caffeine sensitivity. It's when certain factors prompt your body to have adverse reactions to caffeine, even when you're already used to having your fix everyday. This article shows you how to cope depending on the level of sensitivity you have.
Have you been drinking coffee for years and are starting to feel weird sensations after a cuppa?
You’ve got to know something.
If you suddenly find yourself going through unusual post-caffeine effects such as anxiety, headache, faster heartbeat and tremors, you may be experiencing a shift in how your body metabolizes caffeine.
There are three kinds of caffeine consumers - those who experience intense reactions to it at minimal amount, those who can go overboard because having way too much doesn’t really affect them, and those who are somewhere in the middle.
In scientific terms, people are divided according to three levels of caffeine sensitivity:
This is where the majority of the people fall under. With normal sensitivity, they can consume up to 400 milligrams of caffeine in a day and not experience anything unpleasant. 
Caffeine sensitivity is not a random phenomenon that can just happen to any coffee-lover like you. There are a number of factors why you respond to caffeine the way you do:
Certain medications react to caffeine differently. The medicine you take may either amplify or diminish its effects on your body. If you want to find out which medicines have reactions to caffeine, you may check out this article, “Caffeine and medication: can you combine them?”
Genetic link and brain chemistry
The body’s ability to metabolize caffeine depends on your genetic makeup:
The liver breaks down caffeine through an enzyme called CYP1A2. The body produces this enzyme with the help of the CYP1A2 gene, the DNA sequence of which tells how well your body can metabolize caffeine and gets rid of it from the body.  Caffeine sensitivity depends on how much of this enzyme is found in your body.
What regulates the function of CYP1A2 gene has something to do with the AHR gene. 10% of the population has enough AHR gene that causes them to be hyposensitive to the effects of caffeine. 
Caffeine’s effects also depend on the number of adenosine receptors in your brain.“How does caffeine keep you awake” explains exactly what happens when caffeine interacts with your body:
Your body breaks down a high-energy molecule called ATP, which is needed for its constant supply of energy. As it performs this function, it liberates adenosine, a sleep-inducing molecule in your body that causes sleepiness. When adenosine binds to adenosine receptors in the brain, it slows down nerve cell activity, making you drowsy.
Here comes caffeine, which looks like adenosine. As an “adenosine receptor antagonist,” caffeine is recognized by the nerve cells and receptors as adenosine. When caffeine binds into the receptors, it blocks adenosine and inhibits the latter’s effects on your body.
Because caffeine binds to adenosine receptors, its effects depend on the type of receptors found in your brain (correct ones should exist to get the most of your caffeine fix). 
A 2012 study reports that people experience heightened effects of caffeine because of the ADORA2A gene. Those with this gene tend to be hypersensitive to caffeine. 
Pregnancy and taking oral contraception
Caffeine sensitivity becomes more likely when a woman takes oral contraception or becomes pregnant. This is because of higher estrogen levels, which hampers CYP1A2 from fully functioning.
Having a bun in the oven slows down the body’s ability to break down caffeine, so higher levels of caffeine stay longer in the bloodstream. The farther along you are, the longer caffeine stays in your system, and the more intense its effects are in your body. 
What does caffeine sensitivity look like: signs and symptoms
While not necessarily detrimental, caffeine sensitivity may bring about inconvenience and discomfort.
According to Medical News Today and Healthline, here are some signs and symptoms of coffee sensitivity: [9, 10]
The most logical - albeit not the easiest - way to get rid of caffeine sensitivity is to take it easy on your caffeine fix. It makes total sense to try easing off on your go-to caffeine (or rid it altogether if you can).
Be extra cautious on your caffeine intake. It doesn't hurt if you'd start reading the label more and monitor your caffeine mg's. Find out up to what amount of caffeine your body can take and stick to it.
Veer away from highly caffeinated products. Try switching to decaf or have some black or green tea instead.
Ask yourself this existential question - why do I still drink coffee?
But humor aside, since you don't necessarily get the positive effects of caffeine (you seem to be immune in it anyway), assess whether you still get any advantage from getting caffeinated. Otherwise, you may as well switch to decaf so you can steer clear of the adverse effects of consuming caffeine over a long period. 
Erectile dysfunction. In combination, those are two of the ugliest words known to man. But can caffeine help you get it up?
Science hasn't found the definitive answer to this question, but one study concluded that fewer men who consume caffeine have problems performing. The study said:
Caffeine intake reduced the odds of prevalent ED, especially an intake equivalent to approximately 2-3 daily cups of coffee (170-375 mg/day). This reduction was also observed among overweight/obese and hypertensive, but not among diabetic men. Yet, these associations are warranted to be investigated in prospective studies