Although it's been rumoured that the CIA has been working on a truth serum for decades, no such drug has been proven to exist.
Most sources I scanned state that "truth serum" may make a subject more friendly and co-operative with an interviewer, but may also cause the subject to have fantasies. A liar without the serum will likely continue to lie with the serum. These drugs may also make the subject more susceptible to suggestion. The subject may not even remember that an interview had taken place--like the effect of a date rape drug. So the information from drug enhanced interrogations cannot be taken at face value.
In fact, in 1963, Townsend v Sain, the Supreme Court decided that confessions made under the influence of truth serums were "unconstitutionally coerced".
Sadly, it seems that there is no way of forcing someone to tell the truth, even in dire circumstances. However, I join with you, BeginnersLuck, in wishing that somehow, especially when someone is missing, there was a way to compel a suspect to reveal what they know to be true.