Can you tell me about drugs like advair, singulair, (montelukast) viagra..etc..why are they advertised. what drugs aren't?

Lawyers: Basically it used to be unlawful and considered bad form to advertise medication to the public. Pharmaceutical companies, advertisers, etc., went to court and lobbied state & federal governments to eliminate these limitations. Any drug can be advertised- but you want to get a return on your advertising dollar, so more likely it will be an expensive drug that has less expensive competitors.