Almost all of the evidence based research (such as from Harvard/Wellman Center at Mass General, NASA, etc) has been conducted utilizing Class 3B laser and SLED systems.
Unfortunately, most of the lasers utilized in the United States at chiropractic and physical therapy clinics seem to be the high-powered Class 4 Lasers. This is likely due to how much the U.S. Class 4 Laser companies heavily market their products to medical providers and convince them that “higher power is better.” When in fact research and clinical case studies prove that “Less is More”: lower power and longer durations are the keys to success with Low Intensity Laser Therapy.
There has been some discussion that high powered Class 4 lasers actually SLOW DOWN THE TISSUE HEALING PROCESS. They have a short term effect of pain reduction, but in the long run do not work to heal an injury.
Examples of Class 3B lasers are The Bioflex, Microlight, and Thor (which is what NASA uses on the Space Station).
This article provides a good in depth summary of Class 3B versus Class IV Lasers.