Each one has a different effect. LSD is very different from scopolamine. And there are short-term effects, long-term effects, and even side effects to consider. So there is no easy answer. Psychedelic drugs like LSD will cause hallucinations, disorientation, depersonalization, derealization, loss of ego. In short, it can confuse people and cause them to change their reality as to why they are keeping a secret. Simply put, it may make them think their secret is no longer worth keeping. But different people respond differently to LSD. Someone who has never tried it (or another drug for that matter) may be greatly compromised. Whereas someone experienced users like Timothy Leary or Jimi Hendrix may not be as compromised.
Some drugs act on the central nervous system to induce states of extreme anxiety. For example, the venom of the Irukandji Jellyfish is not only deadly, but one of the more nasty side effects is that it can cause and intense feeling of impending doom. Fear can motivate people to tell the truth but it can also cause false positives where the person will tell you anything you want to hear in order to make the torture stop.
Some drugs act like a sedative and/or hypnotic. Did you ever see an internet viral video of a kid after they get their wisdom teeth out and are coming down from the general anesthetic? "Is this real life?" In such a state a person might in such a state of confusion that the truth will simply slip out or they don't even understand that it was a secret.
Sometimes a combination of truth serum drugs can be used. For example you might spike their drink with drug that affects memory such as Versed or a Rohypnol so you can then bring the person to an interrogation room. You might then administer an anxiety inducing drug. Interrogate them them, and then dump them somewhere safe. When the person wakes up, they might not even remember what happened or that they were interrogated at all. Such a tactic might be effective against a mark who regularly abuses alcohol. They might just wake up with a bad hangover and think they got black-out drunk again.
Another method is to use drugs to break their will. While not exactly a truth drug, it will force the mark to be obedient and develop something like a Stockholm syndrome. In illegal human trafficking, they might kidnap someone and force them to use narcotics like heroin so they develop a dependency. Once they are hooked, they are easily controlled.
Depending on how much time you have, one of the more effective methods for interrogation is to use psychological tactics. Play good cop, bad cop, more or less. Put a person under stress for long enough and then send someone else to be their advocate in exchange for information. This is a method that is probably most commonly used in Guantanamo because they can use it with turning it into a black operation. Making someone uncomfortable and tricking them psychologically isn't really against any laws, whereas as beating them and shooting them full of drugs is clearly defined as illegal. It's all about how much time you have.
If I was a CIA spook and had a few weeks/months, I would use the psychological method. If I had only 1 - 2 days and the option to kill them afterwards, I would use extreme psychological methods followed by a chemical or pain/torture method as a last resort. If I had only a few hours I might go straight to the chemical path. If I only a few minutes, (Jack Bauer) I'd go straight to a pain/torture method. In any event, if you use a chemical or pain/torture method, you'll probably have to kill them afterwards.