Such drugs have been utilized by some of the three-letter government agencies in the not-so-distant past (CIA, FBI, DOD, KGB, etc.), particularly during the rampant paranoia of the Cold War. And in the aftermath of 9/11, there was some discussion on the idea of bringing them back into use for interrogation. But are these truth serums effective? Do they produce any useful results?
The short answer is, no. The long answer is "Noooooooooooo!" while running in slow-motion.
Many barbiturates fall under the "truth serum" category, including scopolamine, sodium amytal, and Sodium Pentothal. Scopolamine was tested in the 1950s as a truth serum in project MKULTRA, and is now infamous as a date-rape drug due to its tendency to cause retrograde amnesia (the inability to recall events prior to its administration). Sodium Pentothal is a drug which is commonly used in operating rooms as general anesthesia, though in recent years it has been largely replaced by better alternatives.
Another of the most common truth serums is ethyl alcohol, the same agent that is found in alcoholic beverages. As a truth serum, it is usually injected in a nearly pure form, but its effects are indistinguishable from those caused by consuming large amounts of alcohol orally. If you've ever been intoxicated, then you are personally familiar with the effects that truth serum has on the mind and body.
While a drunk person may be more likely to confess their secrets, they are not incapable of lying, nor will they necessarily share any information that is asked of them. All of these truth serums work in the same manner: They depress the central nervous system and interfere with judgment and higher cognitive function. A person in such a state tends to regurgitate a cocktail of information which is a blend of facts and fantasy, with many details exaggerated or omitted. In a word, unreliable.
In 1963 the Supreme Court ruled that a confession produced under the influence of truth serum was unconstitutionally coerced, and therefore inadmissible. After that, the use of such drugs fell rapidly from popularity in the U.S.. But truth serums may not be gone for good, as the Supreme Court asserted shortly after 9/11 that terrorism may require "heightened deference to the judgments of the political branches with respect to matters of national security."