Very closely chemcially related to Sodium Lauryl Sulfate, this another compound in the class we curlies refer to as Sulfates.  They are detergents and detergents were developed to clean things really well.  Unfortunately, they clean too well for curly hair.  They strip out all of the oils, moisture, and other goodies we so lovingly slather on our hair. 

Sodium Laurelth Sulfate is also known as SLES.  While SLES does not, on it's own cause cancer, it is often contaminated with dioxane, which is a known human carcinogen.  Like the other

It has other health effects too.  It has been linked to a rasied incidence of canker sores (cold sores).  Canker sores are usually due to an infection by the herpes virus.  Once an individual is infected by herpes, the virus never really goes away.  It will retreat to the central nervous system and become dormant for periods of time.  It usually only becomes active when the individual's immune system is not capable of keeping it supressed.  The causes for this vary but stress, sunburn, and other things that supress the immune system.  The logical leap here is that SLES is supressing or disrupting the immune system. 

References - Wikipedia Article, E Medicine.com, Matieral Safety Data Sheet