The initial dose of Celexa is made carefully on an individual patient basis to maintain the lowest effective dose. For many people, finding the correct dose is by a trial and error process. Ideally, 20 mg of Celexa per day is prescribed as the initial dose. A single dose can work well for one person but not at all for someone else. If the symptoms are prominent, your doctor may increase the dose to 40 mg a day after at least one week of treatment from the initial dose.