First trial finds probiotics don't beat placebo for substance-induced depression.

Other research confirms this: a randomized controlled trial gave people with depression from substance use either probiotics or a placebo, and both groups improved equally. So the probiotics didn't provide any extra benefit beyond the placebo effect. That's a clear finding from the first study of its kind for this specific condition. It's important to note that this was a small study, so larger trials might find different results—but for now, the evidence doesn't support using probiotics for substance-induced depression.