For optimal effeciency in the espionage business, the effects of a disorienting drug should never last more than a few hours. If the intent is to get information from the victim, a longer wait can be inconvenient and even fatal. Almost everything done in the trade has the potential for fatality.
Human behavior is unpredictable enough.
Inducing extra unpredictability has to be limited to short term duration. The drug might not have been totally to blame. Going without food and sleep, the waves of electrical storms, and being soaked to his underwear could conceivably have had their own numbing effects on the body's resistances, prolonging his loose grip on reality, and even contributing some curious side effects of their own. It's hard to overestimate the value of dry underwear to peace of mind and coherent thought. It was dark, and he was cold and wet and unable to remember why he was sitting in a puddle in the middle of a weedy, overgrown minefield in Kosovo.