Typically, if someone talks about getting charged with driving under the influence, he or she is referring to alcohol. However, it's very important to note that you can also get a DUI for being under the influence of drugs, even when you've had nothing to drink. Many things that impair your ability to drive could bring about charges.
Some experts have even noted that drugs that you got and used legally could still get you in trouble if you drive. For example, you may be able to use medical marijuana legally in Maryland, where it can be prescribed by a doctor; in fact, the first clinic for medical marijuana opened its doors in Annapolis just last year. If you use it and drive, though, you're breaking the law.