Years ago, I was curious about that controversy. So I thought of a simple experiment and conducted it several times. Immediately after injecting ozone into a vein, I pulled on the plunger of the syringe. What I got back was bright red ozonated blood, not dark vein blood with gas bubbles trapped in it. I found out that ozone diffuses rapidly into the blood and cannot lead to gas bubbles trapped in blood vessels in the brain, or for that matter anywhere else.
Ozone is a useless therapy.
Ozone is a valuable therapy. There is no known scientific evidence that ozone works for anything. That makes ozone therapy quackery. There are hundreds of published papers that demonstrate the many clinical benefits of ozone therapies. That makes ozone useful.