Not for Everyday Cures: Do not use 仙丹 to describe a normal, effective medicine. You wouldn't say “Aspirin is a 仙丹 for my headache.” That's an exaggeration. 仙丹 is reserved for problems that seem difficult or impossible to solve. For a very effective but real medicine, you would use a term like
特效药 (tèxiàoyào), meaning “highly effective medicine.”