Hulda Clark had thousands of patients who did LFs with ES and none of them had a problem. So people who react to ES are rare. It's possible the people who feel ill after doing a Liver-Flush do think sometimes it is because of the ES but it seems more likely that it is caused by the toxins released and adsorbed into the body. This is one reason Clark advised doing Kidney Cleanses before doing LFs.
Telman wrote that the Liver-Flush is more effective using ES than other magnesium laxatives due to the sulfur in the molecule.
Some people have reported that using Dr. Sutter's coke flush or Tsetze's raw egg and lime/lemon is easier to do and gives good results.
H. Clark wrote to keep doing LFs every two weeks until you get 2000 or more stones out. I've read that Moritz wrote that the 7th flush is when the congestion starts to break up and the process really begins to become effective.
If you are worried about an allergic reaction you could put a little under your tongue and see if you have a reaction - do an online search for kind of allergic reactions such as sore throat, runny nose, etc. You can order pure Es online. Sometimes the USP ES in stores contains contaminants.