Hyperbaric Oxygen Therapy Side Effects and What to Expect

Is HBOT safe? The short answer

HBOT given under medical supervision is generally safe, but it is not free of side effects. Mayo says most complications are mild and don't last, and serious ones are rare (Mayo Clinic, 2024). In August 2025 the FDA warned hyperbaric providers that it knows of reports of serious injuries and deaths, including from fires. It said serious adverse events are rare but "can and do occur" (FDA, 2025).

Any page that says HBOT has "no side effects" is wrong.

Common side effects of hyperbaric oxygen therapy

The numbers below come mainly from a review of HBOT side effects in Advances in Wound Care (Heyboer et al., 2017). Rates vary widely between studies, so treat them as rough guides.

A 2023 Cochrane review of HBOT for radiation injury confirmed two of these with high-certainty evidence: a higher risk of reduced sharpness of vision (usually temporary) and a higher risk of ear barotrauma.

Mayo also lists a short-term drop in lung function. Cataracts can form with long courses of treatment. Heyboer notes this usually takes 150 or more treatments, while US courses rarely go past 75.

How side effects are prevented

Your care team will teach you to clear your ears by yawning or swallowing. If ear problems are serious and you need more sessions, they may suggest small ear tubes. People with hay fever or nasal symptoms may be given a decongestant, steroid or antihistamine. If you're anxious in small spaces, you may be offered a medicine to help you relax (Mayo Clinic, 2024).

Rare but serious risks

• Seizures from oxygen toxicity. Too much oxygen in the central nervous system can trigger a seizure. Heyboer puts the rate at about 1 in 2,000 to 3,000 treatments. Older estimates were about 1 in 10,000. After one seizure, the chance of another is under 10%.
• Lung collapse (pneumothorax) and lung barotrauma. Mayo lists lung collapse as an uncommon, more serious complication. Heyboer says lung injury is unlikely and can be avoided with screening before treatment.
• Oxygen poisoning. Cleveland Clinic calls this rare and says severe cases can be fatal (Cleveland Clinic, 2023).
• Fire. Oxygen-rich air makes fire more likely. Certified programs must follow fire-prevention rules (Mayo Clinic, 2024). The FDA's 2025 letter cites fires that caused serious injuries and deaths. It said their root cause was not known.
• Heart strain. A few people with severe heart failure may have heart function problems (University Hospitals).

Who should not have HBOT (contraindications)

An untreated collapsed lung (pneumothorax) is an absolute contraindication. That means you should not have HBOT at all until it's treated (Heyboer et al., 2017). Cleveland Clinic (2023) says HBOT may be dangerous if you have:

• a collapsed lung
• a lung disease that raises the risk of collapse, such as COPD, cystic fibrosis or emphysema
• a fever or a cold
• a recent ear injury or ear surgery

Cleveland also says you shouldn't have HBOT if you're pregnant. Heyboer adds that a cold or other upper respiratory infection is a reason to delay planned treatment. Serious lung disease needs a risk-benefit review, and caution is needed with a weak heart.

Your hyperbaric team should screen you before your first session. Tell them about every lung, ear, heart and sinus problem you have, and every medicine you take.

Why you may feel tired after HBOT

Tiredness after a session is a known effect. Mayo says you may feel somewhat tired or hungry afterward, and that this doesn't limit your usual activities (Mayo Clinic, 2024). University Hospitals says you may feel lightheaded or tired, which usually fades soon. It also warns that sessions can cause extreme fatigue and suggests having someone drive you home (University Hospitals).

None of the medical sources we checked explain why HBOT makes people tired. If the tiredness is severe or doesn't go away, tell your care team.

What to expect: before, during and after a session

Before your session

Your care team will give you a list of rules. The rules exist mainly to prevent fire.

• Clothing. You'll usually change into a hospital gown or scrubs, often 100% cotton. The FDA says some fabrics, such as wool and synthetics, can build up more static electricity (FDA, 2025).
• No hair or skin products. Mayo says not to wear lip balm, lotion, makeup or hair spray.
• No deodorant or perfume. University Hospitals also lists deodorant, perfume, cologne, nail polish and petroleum-based skin products. These are banned because of fire risk in the oxygen-rich chamber.
• Nothing that can spark or heat up. No lighters, matches, phones or battery-powered devices.
• Other items. University Hospitals also asks you to remove jewelry, hearing aids, dentures and hard contact lenses. Tell the team if you use an insulin pump or have an ostomy bag.

In general, don't take anything into the chamber unless a member of your team says it's OK (Mayo Clinic, 2024).

During your session

In a one-person (monoplace) chamber, you lie on a bed that slides into a clear tube filled with pressurized oxygen. In a multi-person (multiplace) chamber, you sit in a pressurized room and breathe oxygen through a mask or hood (Mayo Clinic, 2024).

As the pressure rises, your ears may feel clogged. Yawning, sipping water or swallowing with your nose pinched can help (Cleveland Clinic, 2023). You breathe normally. At University Hospitals, patients can watch TV or listen to music and talk with the therapist at any time. Staff monitor you for the whole session.

After your session

The chamber is slowly brought back to normal pressure. Your ears may pop as it does (MedlinePlus, 2024). At Mayo, the team checks your ears, blood pressure and pulse before you leave.

How long is an HBOT session, and how often?

Most sessions last 1½ to 2 hours (Mayo Clinic, 2024). The UHMS describes 90 to 120 minutes per treatment (UHMS).

How many sessions you need depends on the condition:

• Emergencies. Carbon monoxide poisoning may need only a few sessions (Mayo). A session for decompression sickness may last longer but may not need repeating (MedlinePlus, 2024).
• Chronic conditions. The UHMS says most conditions need treatments daily for several weeks. Mayo says nonhealing wounds may need 40 sessions or more. Cleveland Clinic says sessions are often five days a week, for 30 or more sessions in total.

Your doctor sets the schedule. More sessions also mean a higher risk of side effects such as vision changes.

Mild chambers and home use: different risks

Soft "mild" chambers run at lower pressure than medical HBOT. The UHMS calls mild HBOT (below 1.5 ATA) unproven. It warns that many of these chambers operate in wellness centers and shopping centers without proper fire safety and chamber standards, "putting those exposed at risk for serious injury and even death" (UHMS). Read our mild vs hard-shell HBOT guide before you book one or buy one.

How to choose a safe HBOT provider

Ask these questions before you book:

• Is the chamber FDA-cleared? HBOT chambers are Class II medical devices cleared through the FDA's 510(k) process. You can look them up under product code CBF in the FDA's 510(k) database (FDA, 2025).
• Is the facility accredited? The UHMS accredits hyperbaric facilities against safety and clinical standards (UHMS). Harvard Health notes the FDA recommends HBOT only at an accredited facility (Harvard Health, 2024).
• Who runs the chamber? Look for a Certified Hyperbaric Technologist (CHT), a credential from the National Board of Diving and Hyperbaric Medical Technology.
• Who supervises? UHMS says treatment should be prescribed and supervised by qualified physicians.
• Will they screen you first? A good provider checks your lungs, ears and medicines before the first session.

Prices and coverage vary. See our HBOT cost guide and HBOT insurance coverage guide. To learn what HBOT treats, read what is HBOT.