Bias means systematic error. Its most common form is confounding bias, where various factors in the context of treatment influence the results, without the awareness of clinician or patient. Incorrect claims are made when these confounding factors are ignored. Randomization is the best solution to confounding bias. Clinical examples are provided for antidepressant discontinuation in bipolar depression and for the relationship between substance abuse and antidepressant-related mania. Other forms of bias are discussed, such as measurement bias.