5 best-selling prescription meds of all time

3. Seretide Seretide logged 2011 sales of $8.7 billion, which was its best year ever. The drug is sold by GlaxoSmithKline and is used to treat asthma. Seretide saw generic competition a couple of years ago in many of its global markets, but still holds protection in others, which is helping it hold onto many billions in annual sales. The compound is also said to be difficult to copy, which helps keep Glaxo's original version selling well.

3. Seretide Seretide logged 2011 sales of $8.7 billion, which was its best year ever. The drug is sold by GlaxoSmithKline and is used to treat asthma. Seretide saw generic competition a couple of years ago in many of its global markets, but still holds protection in others, which is helping it hold onto many billions in annual sales. The compound is also said to be difficult to copy, which helps keep Glaxo's original version selling well.