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Imagine a stack of dollar bills reaching from the ground to more than 95,000 miles into the sky -- nearly four-tenths of the way to the moon. That stack would total $1.4 trillion. It's a staggering number. It's also how much will be spent worldwide on medications by 2020, according to a report from QuintilesIMS

That's a lot of money that will primarily go to big drugmakers. And roughly 40% of the total will stem from just seven therapeutic areas. Here are the seven medical conditions that will be lining big pharma's pockets by 2020.  

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No. 7: Viral hepatitis

QuintilesIMS predicts that between $45 billion and $55 billion will be spent globally on drugs that treat viral hepatitis by 2022, reflecting a compounded annual growth rate (CAGR) of 7% to 10%. The World Health Organization (WHO) estimates that around 257 million have chronic hepatitis B virus (HBV) infection and 71 million people have chronic hepatitis C virus (HCV) infection. 

While there are more patients with HBV, it's the HCV market that is generating the most money for big pharma. The current leader in HCV is Gilead Sciences (NASDAQ:GILD). In 2016, the big biotech made $14.8 billion from its HCV drugs. Gilead's HCV revenue is declining, though. The company has seen fewer patient starts, particularly in the U.S., in large part because the sickest patients have been cured. Increased competition from other drugmakers has also led to lower HCV drug prices.

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No. 6: Respiratory diseases

Drugs for treating respiratory diseases are expected to generate between $53 billion and $56 billion in sales by 2020. The most common respiratory diseases include asthma, allergies, and chronic obstructive pulmonary disease (COPD).

Although the global market for respiratory diseases is quite large, there are relatively few huge blockbuster drugs for treating the indications. According to market research firm EvaluatePharma, only one respiratory-disease drug will rank in the 50 top-selling drugs in the world five years from now: Boehringer Ingelheim's Spiriva. The asthma and COPD drug is projected to make $2.7 billion in 2020, down from current sales levels.

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No. 5: Autoimmune diseases

Autoimmune disease medications could make big pharma companies between $55 billion and $65 billion by 2020, according to QuintilesIMS. The therapeutic category is also expected to be one of the fastest-growing areas, with a CAGR of 11% to 14%.

Several large drugmakers compete in the autoimmune disease market. AbbVie (NYSE:ABBV) claims the current top-selling drug, Humira, which made over $16 billion in sales last year. One up-and-comer, though, is Celgene's (NASDAQ:CELG) Otezla. EvaluatePharma projects Otezla will rank as the 32nd biggest-selling drug in the world by 2022, with projected sales of $2.6 billion. 

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No. 4: Cardiovascular diseases

QuintilesIMS estimates that cardiovascular disease drugs will rack up sales between $73 billion and $76 billion by 2020. Roughly one-third of this total could come from what QuintilesIMS refers to as "pharmerging markets." Cardiovascular drugs include treatments for hypertension, heart disease, and cholesterol.

Johnson & Johnson claims the top-selling cardiovascular drug in the world right now with Xarelto, which made nearly $5 billion last year. However, Bristol-Myers Squibb (NYSE:BMY) and Pfizer (NYSE:PFE) appear to be in position to overtake the lead in a few years with their rival anticoagulant, Eliquis. 

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No. 3: Pain

Pain medications are projected to be the third-biggest moneymaker by 2020, with sales of $82 billion to $85 billion. This category of drugs includes treatments for musculoskeletal pain, arthritis, anesthesia, analgesics (both narcotic and non-narcotic), and migraine.

Amgen stands out as one big biotech that could emerge as a major player in the market for pain medications. The company is partnering with Novartis on development of experimental late-stage migraine drug erenumab. If approved, erenumab could reach peak sales of more than $2 billion. 

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No. 2: Diabetes

It's probably not surprising that diabetes drugs are expected to be near the top in spending, with projected global sales between $107 billion and $113 billion by 2020. WHO estimated that there were 422 million patients across the world with diabetes three years ago. The number is probably higher now. 

Novo Nordisk currently claims the greatest market share in the diabetes market, with combined sales of its products totaling over $12.9 billion last year. The Danish drugmaker appears likely to remain at the top over the next few years with diabetes drugs including Victoza, Tresiba, and pipeline candidate semaglutide.  

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No. 1: Cancer

The top medical condition that will likely line big pharma's pockets by 2020 is cancer. QuintilesIMS projects that sales of cancer drugs will be between $100 billion and $120 billion within the next three years. Annual growth is expected to range from 9% to 12%.

Swiss drugmaker Roche (NASDAQOTH:RHHBY) is the clear leader in the oncology market right now. However, Celgene's Revlimid is expected to be the top-selling cancer drug in the next few years. Other big pharma companies on the list for other medical conditions will also make billions from oncology products, including AbbVie, Bristol-Myers Squibb, Gilead Sciences, Novartis, and Pfizer. 

Keith Speights owns shares of AbbVie, Celgene, Gilead Sciences, and Pfizer. The Motley Fool owns shares of and recommends Celgene, Gilead Sciences, and Johnson & Johnson. The Motley Fool recommends Novo Nordisk and Quintiles IMS Holdings. The Motley Fool has a disclosure policy.