I told the pharmacist I need something for my cold.

This joke gets its chuckle from a clever bit of wordplay, specifically a pun that twists a common idiom into a literal action. The humor hinges on the phrase "blow your own horn." Typically, this means to boast about your accomplishments or praise yourself. However, in the context of having a cold, the pharmacist takes the "blowing" part quite literally, offering a tissue for a much more immediate and less metaphorical kind of blowing.

The setup plays on our expectation when someone goes to a pharmacist for a cold; we anticipate advice or medication for symptoms like congestion or a sore throat. Instead, the punchline delivers an unexpected, literal interpretation that subverts those expectations. It's a classic misdirection (Review), taking a familiar phrase and applying it in a surprisingly mundane, yet funny, way that only makes sense because of the shared word "blow."

This kind of joke relies on the audience understanding both the common idiom and the practical reality of having a cold. It's a lighthearted reminder that sometimes, the simplest solution to a problem, especially a runny nose, is to handle it yourself, even if the phrasing makes it sound like you should be patting yourself on the back while doing so.