What about it? Not all speech is protected and there is a case to be made of the harm such speech poses. This is why we can't yell "fire!" in a crowded theater. Schenck v. United States, 249 U.S. 47 (1919)
The bar for prior restraint based on content is pretty high. I believe it's "clear and present danger". You're likely to have a difficult time making the case that ads for allergy medications present a clear and present danger.
The bar for prior restraint on public airwaves is much lower. We'd be much better off if drug marketing were relegated to the shady corners of the internet for advertising.