Is it true that you can only get antivenom once?

Not at all! Modern antivenoms cause very few side effects for most people, even when they get them a second time. In the past, most antivenoms were of the old-fashioned allergy-provoking type, and back then some doctors and many snake experts avoided risking a second use of antivenom. The reason was that a person’s immune system might recognize the animal serum in the antivenom, and there might be either a serious allergic reaction or a severe case of “serum sickness.” But even back then, many people got antivenom a second (or third, or fourth…) time, often by using allergy medications at the same time.