There are the famous stimulant drugs everyone can name with ease: tobacco, cocaine, and the amphetamines. Yet there are some other drugs that many people can become addicted too.
A friend of mine back in high school was unofficially addicted to Dayquil. He would explain that he takes one dayquil every morning to wake up. The stimulant in the non-drowsy dayquil would keep him awake during the morning. The stimulant is sudefedren, which can be used to make methamphetamine. Unofficially, this addiction became a physical dependence and a small psychological dependence. He complained of exhaustion in the morning and headaches when he would skip a day. When he took the dayquil, he never had any symptoms. This fits in fairly well with a physical dependence. He would also complain of a craving to take some if he woke up early to take a test first hour.
This student had a high tolerance for dayquil that ramped up early in the addiction. He started by taking only one in the morning. Then it became two in the morning to get ride of the tired eyelids and body. Very soon he was taking two in the morning, one at lunch, and two before a physical activity such as intramural basketball.
This student did not go to rehab or the doctor to diagnose this addiction. Because of this, everything I have said above is what the student told me and some of my own speculation. It does bring up an interesting topic when we realize that there can be many other addictions and dependences to the less infamous drugs in our lives.