Yes, taking Concerta for several years could potentially slow down metabolism, leading to weight gain. This is because the medication is a stimulant and can affect metabolism. However, weight loss is a more common side effect seen in individuals taking the medication regularly. Additionally, it is possible for a person to gain weight when they stop taking the medication, as the body may struggle to adjust to the changes in metabolism. As long as the medication is taken regularly at the same dose, it is unlikely for a person to experience weight gain due to the medication.