Objective: To determine the relationship between treatment duration and the efficacy of pharmacological treatment for reducing ADHD symptoms. Method: We conducted a systematic review of randomized, double-blind, placebo-controlled clinical trials investigating the efficacy of pharmacological interventions in patients with ADHD. The last bibliographic search was performed in April 15, 2019. The effect of treatment duration on efficacy was studied using meta-regression. Results: A total of 87 studies lasting from 3 to 28 weeks were included. Pharmacological treatment improved ADHD symptom severity by −7.35 points. Treatment duration did not moderate the efficacy of pharmacological treatment. Consistent results were found for psychostimulant drugs, methylphenidate, amphetamine derivatives, atomoxetine, and α2-agonists. A negative correlation was found between baseline ADHD severity efficacy (Coefficient = −.250, p = .013). Conclusion: The efficacy of pharmacological treatment for ADHD remains stable over time. A greater efficacy in more severe patients is suggested.