Ritalin is a medicine used to treat ADHD. The medication can have 2 types of effects, firstly, improvement in concentration and decreased hyperactivity, and secondly, sense of Euphoria. The second effect usually decreases over time but the first one usually does not. The dose of ritalin for maximum benefit also depends on your body weight. So if there has been a recent change in your weight, then the dosage might need to be adjusted. Various medications are also known to interact with Ritalin and decrease it's effectiveness. Concentration itself can also be affected by various factors such as stress, your environment, amount of sleep that you are getting, diet, etc.