So far this year, the top five selling drugs have raked in a combined value of $48 billion. Bear in mind, that at this time last year, combined prescription drug sales in the U.S. were $328.6 billion.

Writing for The Motley Fool, Keith Speights, discusses the top five in a lot more detail. 

Coming in at number one is AbbVie’s Humira. This drug has been one of the years best sellers for several years, so its ranking might not come as a huge surprise. It is expected to bring in $18 billion this year alone. 

“Although Amgen won FDA approval for its biosimilar to Humira in 2016, the drug still isn’t on the market. Thanks to an agreement between AbbVie and Amgen, the biosimilar to Humira in 2016, the drug still isn’t on the market. 

“Thanks to an agreement between AbbVie and Amgen, the biosimilar won’t be sold in the U.S. until early 2023. While Amgen will market the product in Europe beginning Oct. 16, 2018, around two-thirds of Humira’s total revenue comes from the United States.”

Slotting into second place is Celgene’s Revlimid, which is marketed for blood cancer. It made a total of $7 billion in 2016 and is expected to surpass $8 billion this year. Speight has explored three potential reasons for dramatic increase. One being that it increased market penetration, second, that the treatment duration for patients taking the drug for multiple myeloma increased, and finally, Celgene increased the price of the drug. 

He went on to forecast 2018 to be a very successful year, with sales expected to exceed $9 billion. Analysts are even projecting $14 billion annually in 2022. 

Following on from them is Amgen and Pfizer’s Enbrel, which is marketed in the U.S. and Canada. The drug is expected to sell about $7.5 billion this year. 

“Look for Enbrel to stay on the list next year, albeit with lower revenue numbers,” explained Speights. “Pfizer faces biosimilar competition for Enbrel in Europe. Meanwhile, Amgen has cut prices for the drug in the U.S. in the midst of a market crowded with alternative therapies.”

Taking fourth and fifth place are two of Roche’s drugs. The hitting the fourth slot is Rituxan, which has been approved for non-Hodgkin lymphoma, chronic lymphocytic leukemia, and rheumatoid arthritis. This is what gives the drug a unique selling point, as it can be used to treat cancer and an autoimmune disease. This is expected to bring in around $7.5 billion in 2017, despite being expected to fall out of this list next year, largely due to there already being biosimilars for it marketed in Europe, which are expected in the U.S. in the next few years. 

Last on the list is Herceptin which has been approved for breast cancer and gastric cancer. This drug is expected to bring in $7 billion this year. “Like Rituxin, though, Herceptin’s days in the top five could be numbered. On Dec. 1, the FDA approved Mylan’s biosimilar to the drug. Only a few days earlier, the first European biosimilar to Herceptin won approval. Roche is also suing Pfizer to keep another Herceptin biosimilar off the U.S. market.”

Roche has obviously made some strong progress with two of its drugs making the list, but it will be tricky for the company to replace sales of both Rituxan and Herceptin as they begin to fade. 

Speights concludes, “It’s not that any of these are horrible picks for investors, but you need to know the bad as well as the good. Overall, I’d say that the companies with the top two best-selling drugs in the world are also two of the best drug stocks to buy.”