TL;DR: Hyperbaric oxygen therapy is broadly safe, but referring physicians should counsel patients on three main side effect families: middle ear barotrauma (the most common, affecting roughly 9 to 15 percent of patients), transient visual changes, and rare oxygen-related events including central nervous system seizures (well under 0.1 percent at standard treatment pressures). Most adverse events are mild, reversible, and managed by chamber staff. Pre-referral screening for cardiopulmonary risk factors, ear pathology, and seizure history reduces complications meaningfully.
Hyperbaric oxygen therapy is a medical treatment in which a patient breathes 100 percent oxygen at pressures above one atmosphere absolute (ATA), and HBOT side effects are the clinical events that may arise during or after that exposure. The therapy is among the safest in modern medicine, but it carries a small set of predictable risks that every referring physician in Canada should understand before signing a referral. This guide covers the incidence, recognition, and management of common HBOT side effects, drawing on the most current systematic-review evidence and applied to Canadian clinical practice.
Across approximately 18,000 patients pooled in a 2025 systematic review, roughly 15 percent experienced an otologic adverse event during hyperbaric oxygen therapy, while serious events such as oxygen-related seizures and severe pulmonary toxicity remained well under one percent. Canada Hyperbarics works with hospitals and regulated facilities across every province, and the side effect profile described here applies to standard medical-grade chambers operated under accredited protocols.
How common are side effects from hyperbaric oxygen therapy?
Most patients complete a full HBOT course without a clinically significant adverse event. Reported pooled rates from large multi-centre series cluster around 9 to 15 percent for any adverse event, with the vast majority being mild and self-limiting. A 2024 review in Annals of African Medicine categorised HBOT complications into four families: barotrauma, pulmonary toxicity, fire hazards, and claustrophobia, with barotrauma accounting for the largest share of clinical events (Mago, 2024 (PubMed)).
Treatment interruptions are uncommon. In a large Italian cohort of 5,962 patients receiving long-term HBOT for chronic conditions, only 1.49 percent of patients required full suspension of therapy because of middle ear barotrauma. The remainder were either managed conservatively or completed therapy after a myringotomy procedure (Nasole et al., 2019 (PubMed)).
For your referral conversation, the practical takeaway is this: the risk of any clinically meaningful HBOT side effect is comparable to many routine outpatient procedures, and serious events are rare enough that they should not deter referral when the indication is appropriate.
What is middle ear barotrauma and how should you screen for it?
Middle ear barotrauma (MEB) is mechanical injury to the tympanic membrane or middle ear cleft caused by failure to equalise pressure during chamber compression. It is the single most common HBOT adverse event. The 2025 NYU systematic review of 18,284 patients reported a pooled MEB incidence of approximately 15 percent, with 42.8 percent of events graded as mild and 6.4 percent as severe (Voigt et al., 2025 (PubMed)).
The Wallace-Teed grading scale is the standard tool used by hyperbaric units across Canada to classify MEB severity:
| Grade | Otoscopic Finding | Typical Action |
|---|---|---|
| 0 | Symptoms only, no findings | Decongestant, continue |
| 1 | Hyperaemia of pars flaccida | Continue with extra equalising training |
| 2 | Hyperaemia plus mild haemorrhage | Pause one to two sessions |
| 3 | Gross haemorrhage of pars tensa | ENT consult; consider myringotomy |
| 4 | Free blood or fluid in middle ear | ENT review before resumption |
| 5 | Tympanic membrane perforation | ENT management; defer until healed |
Most events at our partner facilities are Grade 1, which resolves without interrupting therapy. The 2019 Italian cohort found that 69 percent of all observed barotrauma events were Grade 1 (Nasole et al., 2019).
Pre-referral screening should ask three questions: Does the patient have a history of difficulty equalising during flights or diving? Is there active rhinitis, sinusitis, or upper respiratory infection? Is there a prior tympanic membrane perforation or recent ear surgery? Patients who answer yes to any of these benefit from an ENT review or a short course of decongestant therapy before initiating HBOT.
How serious is oxygen toxicity in hyperbaric oxygen therapy?
Central nervous system (CNS) oxygen toxicity is the most clinically feared HBOT side effect, but it is also one of the rarest. The classic manifestation is a generalised tonic-clonic seizure during a treatment session at high pressure. The molecular mechanism involves saturation of antioxidant enzymes by elevated partial pressures of oxygen, leading to reactive oxygen species accumulation in vulnerable brain regions and ultimately neural network overstimulation (Manning, 2016 (PubMed)).
Reported seizure rates at modern Canadian protocols (2.0 to 2.4 ATA, 90 to 120 minutes per session, with air breaks) are well under 0.1 percent per session. Most documented cases involve treatments at 2.8 ATA or higher, or absent air-break intervals. Once the chamber descends and the patient breathes air, the seizure aborts and recovery is generally complete.
Although the seizure itself is brief and self-limiting, a 2017 case report in Diving and Hyperbaric Medicine documented a single instance of seizure-associated demand ischaemia and stroke in an octogenarian with cerebrovascular risk factors (Warchol et al., 2017 (PubMed)). This case is the basis for a clinical recommendation that patients with significant cerebrovascular disease, prior seizure history, or active CNS pathology should be referred at lower treatment pressures (1.5 to 2.0 ATA), when this is consistent with the indication.
Other contributors to CNS oxygen toxicity risk include febrile illness, hyperthyroidism, ongoing benzodiazepine withdrawal, and hypoglycaemia. Stable insulin-treated diabetes is not a contraindication, but blood glucose should be checked before each session because hyperbaric oxygen is associated with a small drop in serum glucose during treatment.
What are the pulmonary effects of hyperbaric oxygen therapy?
Pulmonary oxygen toxicity is dose-related and rare in clinical hyperbaric exposures. Symptoms include retrosternal burning, cough, and reduced vital capacity, typically only after cumulative oxygen exposures exceeding the standard 30 to 40 session course. For a healthy patient on a typical HBOT protocol, clinically meaningful pulmonary toxicity is essentially never observed, though serial spirometry is part of routine screening at most Canadian facilities.
The more pressing pulmonary concern is barotrauma in patients with bullous emphysema, untreated pneumothorax, or significant air trapping. Pneumothorax during decompression is a true HBOT emergency. Pre-referral chest imaging should be reviewed for any known emphysematous bullae greater than two centimetres, and patients with active or recent pneumothorax must be deferred until full radiographic resolution.
Which patients are at higher risk of HBOT side effects?
The 2025 NYU systematic review identified five risk factors for middle ear barotrauma that referring physicians can quickly assess at the point of referral:
| Risk Factor | Pre-Referral Action |
|---|---|
| Age greater than 60 | Counsel on equalising; flag for slower compression |
| Female sex | Mild risk increase; no specific action required |
| Head and neck pathology or radiation | ENT review before HBOT |
| Sensory neuropathy | Document baseline; ensure clear communication protocol with chamber staff |
| Prior difficulty equalising ear pressure | Decongestant trial; consider prophylactic ventilation tubes |
A 2025 South Korean study of monoplace HBOT identified two additional independent risk factors for middle ear barotrauma: altered mental state at presentation (odds ratio 2.50) and being in the emergency treatment classification group (odds ratio 6.75). Patients receiving emergent HBOT for carbon monoxide poisoning, decompression sickness, or arterial gas embolism warrant particular attention to ear management protocols (Lee et al., 2025 (PubMed)). The single most powerful protective factor identified across the systematic review evidence was experience with effective Valsalva or Toynbee equalising techniques, which can be taught in a five-minute pre-treatment session.
How should referring physicians manage common HBOT side effects?
Most management decisions during therapy are made by the hyperbaric physician on site, but referring physicians often field follow-up questions or see patients between sessions. Use this practical sequence:
- Patient reports ear pain after a session. Confirm timing relative to compression. If pain resolves within minutes of completion, this is likely Grade 0 to 1 MEB. Suggest pseudoephedrine 60 mg before next session and reinforce equalising technique.
- Patient reports muffled hearing or fullness. Otoscopic examination is appropriate. Effusion or visible blood requires the chamber facility to be informed, with possible ENT review before resumption.
- Patient reports transient blurry vision or difficulty reading. Reversible myopic shift is common after 20 to 30 sessions due to lens shape changes. It typically resolves within four to six weeks of completing therapy. Avoid new spectacle prescriptions during this window.
- Patient reports anxiety or claustrophobia in the chamber. Most chambers offer monoplace acclimatisation runs. Low-dose lorazepam pre-treatment is occasionally used, but discuss with the hyperbaric facility first because some sedatives lower the seizure threshold.
- Patient with diabetes reports unusual fatigue post-session. Likely transient hypoglycaemia. Recommend a small carbohydrate snack 30 to 60 minutes before each session and a post-session glucose check.
When does an HBOT side effect require urgent assessment?
Most HBOT side effects do not need emergency intervention, but a small group of red-flag presentations warrants immediate attention. Refer urgently if your patient experiences any of the following after a session:
- New focal neurological deficit suggesting stroke or seizure-related ischaemia
- Sudden chest pain with shortness of breath suggesting pneumothorax
- Acute hearing loss not present at session start
- Active otorrhoea suggesting tympanic membrane perforation
- Persistent post-session confusion in an older adult
For full clinical detail on absolute and relative contraindications, see our conditions index and the UHMS-aligned approach summarised in our regulatory and safety overview. The Undersea and Hyperbaric Medical Society maintains the most current international consensus on contraindications and indications at uhms.org.
What pre-HBOT counselling should referring physicians provide?
A focused five-minute referral conversation reduces complications and patient anxiety. We suggest covering five points:
- Equalising technique. Demonstrate Valsalva, Toynbee, and the modified Frenzel manoeuvre. Most chamber facilities reinforce this at the first session, but prior exposure improves first-session outcomes meaningfully.
- Reversible visual changes. Set the expectation that mild myopic shift is normal after 20 to 30 sessions and resolves spontaneously.
- Hypoglycaemia awareness for diabetes. Recommend a light carbohydrate-containing meal 30 to 60 minutes before each session.
- Smoking cessation. Tobacco use blunts the therapeutic oxygen effect and increases pulmonary risk. A pre-HBOT smoking-cessation conversation is high-yield.
- What to report. Tell patients which symptoms require immediate communication to chamber staff (severe ear pain, chest pain, sudden vision loss, neurological symptoms) versus what can wait until the next visit.
Patients who arrive informed are easier to treat, more compliant with the full course, and more likely to complete the prescribed protocol. Your referral conversation has measurable downstream value at the chamber.
Frequently asked questions about HBOT side effects
Is hyperbaric oxygen therapy safe for older adults?
Yes. Age over 60 is a mild risk factor for middle ear barotrauma but is not a contraindication. A 2022 review in Redox Biology noted that HBOT has a favourable safety profile in older patients across multiple conditions, with appropriate screening and protocol adjustment (Fu et al., 2022 (PubMed)). Slower compression rates and proactive equalising training are routinely used in older adults.
Can HBOT cause permanent hearing loss?
Persistent hearing loss after HBOT is rare. Most barotrauma-related hearing changes are transient and resolve as middle ear effusion clears. Severe Grade 4 to 5 events with tympanic membrane perforation can leave residual conductive deficits, but these are uncommon (under one percent in pooled series) and typically heal with appropriate ENT management.
Should HBOT be avoided in patients with seizure disorders?
Active uncontrolled epilepsy is a relative contraindication. For a patient with stable, well-controlled seizure disorder, HBOT can usually proceed at lower treatment pressures (1.5 to 2.0 ATA) with anticonvulsant compliance verified before each session. Discuss the specific case with the receiving hyperbaric physician.
What is the most common reason patients stop HBOT early?
Patient choice and travel burden are more common than medical complications. Among medical reasons, severe middle ear barotrauma that fails decongestant management is the leading cause of withdrawal, followed by claustrophobia. In the 5,962-patient Italian cohort, only 1.5 percent of patients required complete therapy suspension because of barotrauma.
Are there long-term side effects from completing a full HBOT course?
Long-term side effects from a standard 30 to 40 session course are not well documented in the literature. Cataract progression has been reported after exceptionally long exposures (over 150 sessions), and reversible myopic shift normalises within weeks of completion. Standard Canadian protocols do not approach the cumulative dose at which long-term ocular effects have been documented.
How should I document HBOT-related adverse events for the referring chart?
Most chamber facilities provide a session-by-session adverse event summary on completion. Note the specific event, severity grading, intervention applied, and any modification to subsequent sessions. This documentation supports any future referrals and clarifies which contraindications, if any, apply for repeat HBOT in the future.
Where can I confirm contraindications for a specific patient?
The Canadian Undersea and Hyperbaric Medical Association (cuhma.ca) and UHMS publish current contraindication lists. Our frequently asked questions page covers patient-facing screening points, and our coverage page outlines which chamber types and protocols are reimbursed across Canadian provinces.
Refer with confidence to accredited Canadian chambers
Hyperbaric oxygen therapy maintains a strong safety profile when delivered at accredited Canadian facilities under modern protocols. The clinical reality is straightforward: most side effects are mild, reversible, and predictable from pre-referral screening. With a brief equalising-technique conversation and a focused review of cardiopulmonary risk factors, your referral conversation can prevent most events that would otherwise interrupt therapy.
To find hospitals and regulated facilities near your patient, including those that accept provincial-coverage referrals, browse the Canada Hyperbarics facility directory by province. For condition-specific evidence summaries to support your referral, see our research database of peer-reviewed studies.
Medical disclaimer: This content is for informational purposes only and does not constitute medical advice. Clinical decisions, including HBOT referral and contraindication assessment, should be made on a case-by-case basis by qualified hyperbaric medicine specialists in collaboration with the referring clinician. Canada Hyperbarics is an information resource and does not provide medical care.