In vitro studies have demonstrated that a chloroform extract of khat leaves is cytotoxic in cultures of KB, , and XP2B1 mammalian cells. This cytotoxicity appears to be because of inhibition of de novo RNA synthesis affecting all the cell strains tested; KB cells possessed some resistance to the toxicity. 40 The tannins found in khat leaves have been shown to thicken the mucosa of the oropharynx and esophagus and also may be carcinogenic. 41 Oral cancers in certain regions of Saudi Arabia have been found to occur mainly among patients who had been chronic khat chewers. 42 Esophageal and gastric carcinoma have been attributed to khat chewing and water-pipe smoking in men and women in Yemen. 41 Khat-chewing men living in the area of the horn of Africa were studied and were found to have an increased risk of oral carcinoma, especially when accompanied by alcohol and tobacco consumption. 43
Sometimes, putative medical adverse effects are regarded as controversial and generate heated discussions in society and lawsuits against drug manufacturers. One example is the recent controversy as to whether autism was linked to the MMR vaccine (or by thiomersal , a mercury -based preservative used in some vaccines ). No link has been found in several large studies, and despite removal of thimerosal from vaccines a decade ago the rate of autism has not decreased as would be expected if it had been the causative agent.  
Anxiety, gut problems and allergies very often go hand in hand and can be very distressing. Often the root cause of the problem can be found in a damaged gut flora and a weakened gut lining which disturbs the immune system. Research has shown that probiotics has been very beneficial for those suffering from different kinds of allergies and intolerances. Serious probiotics adverse effects are extremely rare. A study in Jan 2013 showed that 8 weeks of eating probiotics decreased the level of allergic markers in the body. That study dealt with pollen allergy but the same applies to many other allergies. The study reported no side effects whatsoever. Many other studies show similar results with probiotics for allergies, depression, inflammation and gut problems. That includes basically all symptoms that you mention. One reason why probiotics works so well for all those symptoms is that the root cause is a damaged/abnormal gut causing a chain reaction that can results in many different symptoms.