HBOT side effects and risks: what to expect and who should not have it

The most common side effect of hyperbaric oxygen therapy (HBOT) is pressure in the ears and sinuses while the chamber pressurises. Temporary short-sightedness, tiredness and claustrophobia are the other effects people notice. Serious complications, such as an oxygen seizure or lung injury, are rare, and their risk rises with treatment pressure and length of exposure.

HBOT carries real risks, and no honest provider should suggest otherwise. What keeps it well tolerated is screening before treatment, careful pressurisation, and an operator who can slow down or stop a session. This guide covers each side effect, how often it is reported, and who should not have HBOT at all.

What are the most common HBOT side effects?

Ear and sinus pressure. Gas in an enclosed space shrinks as pressure rises, so the air in your middle ear and sinuses compresses as the chamber pressurises. It feels like an aircraft descending. You clear it by swallowing, yawning or gently blowing against a pinched nose, and the operator slows or pauses pressurisation if you cannot.

When pressure is not equalised, the eardrum and middle ear can be irritated. This is called middle ear barotrauma. A 2017 review of HBOT side effects found reported rates ranging from about 2 percent to 84 percent, depending on how studies defined and looked for it, with one large recent series reporting 43 percent. Most cases are mild. Nosebleeds from sinus pressure are rare, at around 1 per 10,000 treatments. A cold, hay fever or sinus infection makes clearing harder, so tell the clinic if you are congested on the day.

A 2023 meta-analysis pooled 24 randomised trials with 1,497 participants. Adverse effects were reported by 30.11 percent of people having HBOT and 10.43 percent of controls, and ear discomfort was the most frequent. The review found adverse effects were more common when chamber pressure was above 2.0 ATA and when courses ran beyond 10 sessions.

Temporary short-sightedness. Breathing oxygen under pressure can change the focusing power of the lens. The 2017 review reports short-sightedness in 25 to 100 percent of patients after several weeks of treatment at 2.0 ATA or higher. It usually returns to baseline within 3 to 6 weeks of finishing, and occasionally takes 6 to 12 months. A Cochrane review of late radiation injury trials also found high-certainty evidence of a higher risk of reduced visual acuity with HBOT. If distance vision blurs, do not drive until it is clear again. Cataract formation has been reported mainly after very long courses, usually 150 treatments or more.

Tiredness and light-headedness. Patient information on HBOT, including guidance summarised by the ME Association, lists fatigue and light-headedness after sessions. It is usually short-lived. Plan a quiet hour after your first appointment until you know how you respond.

Claustrophobia. Any chamber is an enclosed space. The 2017 review puts confinement anxiety in single-person chambers at 8 events per 10,000 treatments. At HBOT Medics you remain in contact with the operator throughout, and a session can be stopped. Pressure is still released gradually when a session ends early.

Low blood sugar in people with diabetes. Blood glucose can fall during treatment. One study cited by the 2017 review found blood sugar below 70 mg/dL (about 3.9 mmol/L) in 1.5 percent, with symptoms in 0.19 percent. If you use insulin or other medicines that lower blood sugar, list them on your declaration.

What are the rare but serious risks of hyperbaric oxygen?

Oxygen toxicity seizures. Breathing concentrated oxygen under pressure can, uncommonly, trigger a seizure. The 2017 review puts recent estimates at roughly 1 in 2,000 to 3,000 treatments, with published rates ranging from 1 in 1,651 to 1 in 40,339 depending on the protocol, and it notes that higher treatment pressure increases the risk. The StatPearls review of hyperbaric complications describes the risk as dose-dependent. A high fever or epilepsy lowers the seizure threshold, which is why both are screening questions.

Lung oxygen toxicity. Long exposure to high oxygen levels can irritate the lungs. StatPearls describes this as a function of how long the exposure lasts, developing more quickly at higher pressures.

Lung barotrauma. In healthy lungs, air moves freely in and out as pressure changes, and lung injury is not expected. The problem is trapped air. An untreated pneumothorax, a collapsed lung, can become a tension pneumothorax as pressure changes, which is why it is the one absolute contraindication. Conditions that trap air, such as some forms of COPD, asthma or severe emphysema, need individual review.

Fire. Oxygen does not burn by itself, but it makes other materials burn far more readily. The UHMS warns that facilities operating without proper adherence to fire safety and chamber construction standards put people at risk of serious injury and even death. That is why every hyperbaric centre, ours included, has strict clothing and prohibited-item rules. They are a safety system, not fussiness.

Who should not have HBOT?

The StatPearls review of HBOT contraindications states that the only absolute contraindication is an untreated pneumothorax. Everything else in the table below is a reason for individual review, not an automatic refusal.

Situation	Why it matters
Untreated pneumothorax (collapsed lung)	Absolute contraindication: trapped air can expand dangerously as pressure changes
COPD	Relative contraindication because of the risk of carbon dioxide build-up
Asthma or severe emphysema	Air trapping can cause lung barotrauma
Heavy cold, sinus or upper respiratory infection	Raises the risk of ear and sinus barotrauma; postponing is common
Recent ear or chest surgery	Equalising may be difficult and lung risks need assessing
High fever or epilepsy	Both lower the seizure threshold
Implanted devices	The manufacturer needs to confirm the device is pressure tested
Some chemotherapy drugs, including doxorubicin, bleomycin and cisplatin	Specific safety considerations with oxygen under pressure
Pregnancy	Traditionally a relative contraindication; needs an individual medical decision
Severe claustrophobia	May make a session impractical

Our guide to who HBOT is not suitable for explains how each of these is handled at HBOT Medics.

Is hyperbaric oxygen therapy safe at lower pressure?

Lower pressure reduces some risks but not all of them. The evidence above links oxygen seizures and lung oxygen toxicity to higher pressure, and the 2023 meta-analysis found more adverse effects above 2.0 ATA and in courses beyond 10 sessions. Our chamber operates at 1.5 ATA, below the pressures most of those figures come from.

That does not make screening less important. Ear and sinus pressure happen whenever pressure changes, and an untreated collapsed lung is dangerous at any hyperbaric pressure. The contraindications do not disappear because a chamber runs lower. You can read more about the equipment on our chamber page.

How does screening work at a doctor-led service?

Screening at HBOT Medics is built around one medical declaration that covers your course. It is sent around 24 hours before your first session, and it asks about your lungs, ears and sinuses, medicines, current treatment and pregnancy. Every declaration is reviewed before treatment. Answers that raise a concern are escalated for clinical review, and screening standards and governance sit under the service's medical lead.

A flagged answer does not automatically mean refusal. Depending on the issue, it can lead to a follow-up question, postponing until a temporary problem such as a cold settles, or advice that HBOT is not appropriate for you. Honest answers are what make this work, so list anything you are unsure about rather than judging it yourself.

Before every session the operator explains how to clear your ears, how to communicate from inside the chamber, and how a session is stopped. Our session guide and what happens during treatment walk through the hour step by step.

During or after a session, tell the operator or seek medical advice if you have ear pain that does not clear, a change in hearing, blurred vision, twitching or feeling unwell. Chest pain or severe breathlessness needs urgent help: call 999.

If you are weighing up HBOT, the Course includes a doctor consultation, a separate conversation about your history and goals. No referral needed. Medical consultation included. When you are ready, you can see available times.