The sensitivity to spicy herbs usually means they are killing pothogens. As the pathogens die they release toxins, and people have reactions to these toxins. Cayenne can give people with ulcers a bad reaction though, so one should start with a small amount of cayenne and gradually increase it. Cayenne can also produce profuse sweating if you aren't used to it or take too much. While the profuse sweating isn't dangerous(and may even be beneficial) it can sometimes be very uncomfortable.