What does it mean for an intervention to “work”? How do we evaluate efficacy? Are the evaluations for supplements and other non-medical products as rigorous as the evaluations for medications that we prescribe? These are important questions in an age where everyone seems to have a product with generous claims and pseudoscience is becoming too common. We will review the behavioural pharmacology literature with the aim of demonstrating why, if we wish for our clients to trust us and regard us as experts, we must understand what works and what doesn’t and be able to share that information.