15. Copaxone
> 2014 sales: $4.8 billion
> Treatment for: Multiple sclerosis
> Drug manufacturer: Teva Pharmaceutical Industries (NYSE: TEVA)
> Company’s headquarter: Israel

Multiple sclerosis (MS) is a disease that disrupts the transmission of nerve signals from the brain to the rest of the body, leading to a host of potentially debilitating symptoms. Though the disease is relatively rare, Teva Pharmaceutical Industries sold $4.8 billion of its multiple sclerosis drug Copaxone in 2014, up slightly from $4.7 billion the previous year. Competition from generic versions of drugs is inevitable. However, despite the release last year of a generic version of Copaxone from competing pharmaceutical company Novartis, sales of Copaxone remain high.

14. Januvia
> 2014 sales: $5.0 billion
> Treatment for: Diabetes
> Drug manufacturer: Merck & Co. Inc. (NYSE: MRK)
> Company’s headquarter: United States

Diabetes is one of the most common diseases in the United States, afflicting approximately 9.3% of the population, and several treatments of the condition are big sellers. Pharmaceutical giant Merck reported $5 billion in sales of its diabetes drug Januvia in 2014. Like many drugs, however, Januvia has some unforeseen negative side effects. In 2015, the FDA added a warning to the drug that it may cause severe to disabling joint pain. From its introduction in 2006 through 2013, Januvia was linked to 28 cases of debilitating joint pain.

13. Spiriva
> 2014 sales: $5.5 billion
> Treatment for: Chronic obstructive pulmonary disease (COPD)
> Drug manufacturer: The Boehringer Ingelheim Group
> Company’s headquarter: Germany

COPD is a blanket term for a number of respiratory problems, including emphysema, chronic bronchitis, and some forms of asthma, that affect more than 24 million people in the United States alone, according to the COPD Foundation. Spiriva, the most popular medication used to treat COPD, racked up $5.5 billion in global sales in 2014. The drug is marketed and sold by international pharmaceutical company Boehringer Ingelheim, a German drug maker that focuses on respiratory conditions.

12. Herceptin
> 2014 sales: $5.6 billion
> Treatment for: Various cancers
> Drug manufacturer: The Roche Group
> Company’s headquarter: Switzerland

Used to treat a variety of cancers, Herceptin is among the best selling drugs in the world. The drug was one of three by pharmaceutical giant Roche to post at least $5.6 billion in sales in 2014. Like many other popular prescription drugs, Herceptin is not without some potentially serious adverse side effects. A recently published study in the Journal of Clinical Oncology found that Herceptin, while known to increase the risk of congestive heart failure in elderly patients, may also cause heart problems in patients of all ages.

11. Lyrica
> 2014 sales: $6.0 billion
> Treatment for: Nerve disorders
> Drug manufacturer: Pfizer Inc. (NYSE: PFE)
> Company’s headquarter: United States

Since its introduction in 2006, more than 9 million Americans have used Lyrica to treat pain from a range of diseases, including diabetes, shingles, and fibromyalgia. Invented in a lab at Northwestern University, the compound pregabalin is now sold and marketed as Lyrica by pharmaceutical giant Pfizer in an agreement with the university. Sales of the drug hit $6 billion in 2014, up 19% from the year before. Over the years, Northwestern has added $1.4 billion to its endowment from Lyrica sales. While the company’s patent on the drug has already expired in some European countries, Pfizer has exclusive rights in the United States until 2018.