This chapter describes the chemistry of polycavernoside C (PC) and polycavernoside C2 (PC2). The structures of PC and PC2 provide information for the biosynthetic pathway or metabolism of polycavernosides, which is necessary for monitoring the occurrence of the human lethal toxins. The analogs that possess an isopropyl group in the side chain showed high-level toxicity. Pharmacology of polycavernosides was poorly investigated due to the limitation of sample supply. The complete mechanism of action and molecular targets of these new phycotoxins have not been characterized. Karlotoxins provide an interesting opportunity to create a nontoxic cholesterol pharmacophore that could potentially transport cholesterol from the arteries to the liver or kidneys for excretion. Some of the toxins produced by cyanobacteria are neurotoxins that block neurotransmission by acting on cholinergic receptors. The symptoms of intoxication induced by polycavernoside A in mice are similar to a stimulation of the parasympathetic nervous system.