Caffeine is a considered a safe ingredient. It is a stimulant that excites the nerves cells of the brain. While some stimulants, such as nicotine, are considered addictive, you aren’t likely to become addicted to caffeine if you consume it in moderation. But if you have caffeine regularly, you might feel like you're dependent on it. Without it, you might get a headache or feel fatigue, difficulty concentrating, nausea, and muscle pain.