I'm not sure how a chemical injected into the veins will make a person suddenly honest.
I can see how it might make a person less inclined to lie out of fear or something else that can be tempered, but if the subject has convinced themselves of a certain version of events, they will state that truth as casually as if they were sober, I think.
At best, it's a case of what a person would say while intoxicated, i.e. it isn't their true testimony. If you didn't say it while being of sound mind, is it really your testimony? It's like telling a kid "it's opposite day, would you push your little sister in front of a truck?" And they say yes and now you're saying they're crazy when that's not at all what they told you.
Here's a thought experiment. Give an appropriate dose of this to Trump, and ask him who's ruining the country. Dollars to doughnuts he's not going to say he is, he's going to say Democrats and illegal immigrants. That's his truth, he believes it, and he'll say it under the influence of a narcotic substance, I'm sure.