You can also try and read the following article:
Effect of Modulated TENS on Muscle Activation, Oxygenation, and Pain: Searching for a Physiological Mechanism
Authors: Natasa S. Kavcic a; Greg H. Lehman a; Stuart M. McGill a
Also in the following article: Prospective, Randomized, Placebo-controlled Study of the Effect of TENS on Postthoracotomy Pain and Pulmonary Function by Mükerrem Erdogan et al. there was a significant decrease in the need for opiods during the 5 day post-op period with no evidence of side effects or intollerance in the research group.
Nonpharmacological Treatments for Musculoskeletal Pain
*Anthony Wright, Ph.D., and †Kathleen A. Sluka, Ph.D.
' In contrast, Marchand et al.8 showed
that high-frequency TENS treatment significantly reduced
the affective component of pain"
"However, it is difficult to draw
conclusions regarding effectiveness because different
pain syndromes, different stimulation parameters, and
different outcome measures were used in each study."
"...improves the
physical and mental component summary on the Short
Form–36 quality of life survey in patients with chronic
low back pain"
Just a few quotes out of above mentioned article.
The main thing to remember is that TENS will never be used on it own and that other modalities and techniques should also be included in the treatment.