Several side effects and complications from hyperbaric oxygen (HBO2) therapy have been described, with varying degrees of seriousness. By far, the two most frequent and benign side effects comprise middle ear barotrauma, which has been noted in up to 2% of treated patients, and can be prevented or minimized by teaching autoinflation techniques, or by inserting tympanostomy tubes. Another frequent complaint is claustrophobia, both during multiplace and monoplace chamber compression, requiring reassurance, coaching and, at times, sedation. Other more rare, but more severe side effects derive from oxygen (O2) toxicity, from the multiple exposures required for chronic treatments, especially progressive myopia, usually transient and reversible after stopping HBO2 sessions, or pulmonary dyspnea, with cough and inspiratory pain. More serious O2-induced seizures happen rarely, at higher O2 pressures, and often during acute treatments in acidotic patients (carbon monoxide poisoning).