I think it is derived from arsenic, but that doesn't make it bad for you.
Lots of homeopathic remedies are from things that are poisonous in larger doses, because many toxic compounds do a lot of good in smaller doses.
Homeopathic remedies work by irritating the system in order to ramp it up, get its defenses up, much like a smallpox vaccine - it's a cowpox that is easily taken care of by the immune system. It irritates the immune system enough to respond, and from then on it knows how to respond until you get NO symptoms, no pox, again, and thus if you come into contact with smallpox you won't get sick. This is the "magic bullet" syndrome discovered by the doctor who studied snake bite victims in India. People who are bit once and survive don't suffer from a second bite because their immunities are built up.