If you research HBOT on the internet, you will find a lot of positive comments and anecdotes recommending it as effective treatment for a large number of conditions. You will also find negative comments, including a statement from the FDA (Food and Drug Administration) saying that HBOT has not been proven safe and effective for many of the conditions it is recommended for on various websites, and warning people to be careful about any use of HBOT for conditions not approved by the FDA, i.e. for "off-label" conditions. So who to believe?
To understand the situation, I must start with an explanation of the role of the FDA, which is responsible for regulating medications. In 1972 the FDA took on regulation of HBOT, so first of all it is important to understand that HBOT is regulated in exactly the same way medications are regulated, following the same rules.
When a new medication is developed, the manufacturer carries out rigorous double-blind-placebo-controlled trials to determine if the medication is safe and effective. For example, in the case of an anti-depressant, the trial will involve a group of patients who are diagnosed with depression, give half of them the medication, and half of them a placebo (an identical pill that has no active ingredients). The "double blind" part of this means that neither the patient nor the investigator who interacts with the patient to collect the data knows if that patient is getting the medication or the placebo. Various objective measures are used to determine how much improvement comes about from the treatment. If the investigators can show that the treatment group got statistically better than the placebo group, and there is no evidence of unacceptable side-effects, then the FDA may approve that medication for use in that condition - i.e. the medication is now FDA-approved for use in that condition.
As doctors begin to develop experience with the medication, they notice that it is effective in other conditions - for example, anxiety, or maybe PMS (pre-menstrual syndrome). The sequence of events is usually something like this: let's say a doctor notices a depressed patient gets better on the new medication, and also experiences an improvement in anxiety. He finds this interesting, and tries it in a few more patients. If he becomes convinced that it is useful, he might send a letter to a medical journal reporting his observation. Other doctors read the letter, and give it a try with their difficult anxiety patients. After a while, a doctor in a large hospital or university setting, who has some resources, might publish a "case series". That is, he collects data in a systematic way on, let's say, 20-30 patients, maybe does some rating scales to make it more objective, does a little statistics to make it more scientific, and it gets published in a journal. After a few years somebody - usually the manufacturer, who sees the chance to make money - will pay for a formal double-blind-placebo-controlled trial on a group of anxiety patients. (These trials are very expensive). If it is positive, the FDA will eventually approve the medication for treatment of anxiety - another FDA-approved indication. What needs to be understood is that by the time the FDA approves the medication for anxiety, the community of doctors treating that type of patient have been using it routinely for anxiety for years - i.e. they have been using it "off-label" for a non-FDA approved. condition.
The above scenario happens when the medication is found to be effective for a common condition where there is enough interest, and enough money to be made, to justify doing the expensive studies required by the FDA before they will approve that condition. But doctors in clinical practice may have found a dozen uses for the medication for less common conditions, which will never be tested rigorously, and will never be submitted to the FDA. The FDA recognizes that practicing clinicians learn of all sorts of uses for medications that are never submitted for FDA approval, and they state explicitly in their regulations that a physician may use any medication for any purpose that he deems appropriate, regardless of whether or not the condition is FDA approved. Obviously the doctor should have some justification, either in the literature and/or in his experience and the experience of his colleagues. The important point is that the existence of FDA-approval for use of a drug in specific conditions does not in any way imply that a doctor may not use it for non-FDA approved conditions.
In reality there is adequate evidence for HBOT's efficacy in many of these conditions, and there is extensive clinical experience attesting to its efficacy in many conditions for which formal studies have not been done. Many of the conditions for which HBOT has been found to be effective are devastating to the lives of the people suffering from them. On the other hand, when done properly HBOT - particularly mild HBOT- is about as safe as any medical condition can be. The decision whether to undertake HBOT should be based on the risk/benefit analysis, just as the decision to use any medication is based on a risk/benefit analysis. When the potential benefit for a patient suffering from a devastating condition is weighed against the generally mild side-effects and very low risk of HBOT, the majority of patients elect to start treatment, even in the absence of rigorous proof. In short, if the risk of harm is very low, and the potential for benefit is substantial, it is reasonable to proceed.