“For a long time, over-the-counter decongestant products like Sudafed had pseudoephedrine in them. Pseudoephedrine is effective at reducing sinus congestion during colds but is also an ingredient that can be used to make methamphetamine. As a result of abuse of pseudoephedrine-containing products by manufacturers of methamphetamine, federal legislation was passed moving those products behind the pharmacy counter, which made them harder for people to get ahold of for routine use. Since companies would prefer to sell products that people can just easily grab off the shelves, Sudafed and other well-known brands introduced new formulations with another very old over-the-counter active ingredient, phenylephrine. The problem is that phenylephrine doesn’t work, as the FDA finally concluded, unless you directly spritz it onto your nasal passages as with Afrin or products like that.”
https://news.harvard.edu/gazette/story/2023/09/why-are-ineff...