The concept of dosage of hyperbaric oxygen therapy (HBOT) derives from the definition of HBOT as a drug. Using the broad definition of HBOT by Harch and Neubauer (1), HBOT is the use of greater than ambient pressure oxygen as a drug to treat basic pathophysiologic processes/states and their diseases. Drug dosage of HBOT, therefore, is a function of baseline or reference ambient pressure, depth of pressurization, duration, frequency, air breaks, surface interval, number of treatments, idiosyncratic genetic patient factors, and time to intervention in the disease process which determines the pathological targets. All of these factors cause HBOT to be a narrow-window drug in chronic brain injury similar to digoxin and coumadin: too litfie maybe ineffective and too much can be toxic. In addition, oxygen is a respiratory metabolite: ;too little has serious metabolic consequences and too much can cause metabolic fatigue. Determining the proper dosage in a given patient with a specific or multiple diseases can be difficult. Ultimately, one wants the best dosage that improves the patient while doing the least harm.