Whether illicit, prescribed, or over-the counter, all drugs can cause side effects that upset brain chemistry and alter mood, perceiving, thinking, behavior, and sleep. This likelihood increases especially when drugs are combined. Particularly among the elderly, who are often on several medications, a drug cocktail may be an invitation for all sorts of psychiatric mayhem, including anxiety.
Certain drugs also create dependency, both psychological and physical. Withdrawing from them, even slowly, can create unbearable anxiety; some people contemplate or attempt suicide.
These problems are serious. According to Dr. Mark Gold, biopsychiatrist and author of The Good News about Panic, Anxiety & Phobias, drugs may be the most common and insidious of all anxiety mimickers.
Since both therapeutic medications and illegal drugs are so widespread, since the symptoms they produce are so convincingly mental, and since so many psychiatrists fail to investigate thoroughly for drugs before diagnosing, I believe that drugs are the most dangerous and common of all (anxiety) mimickers (p. 176).

Information taken from Anxiety: Hidden Causes