It's no secret that many big pharma companies are highly profitable. In fact, the industry's gigantic profits have attracted plenty of negative attention from politicians and the press. For investors, though, stocks for businesses that are very profitable are a great thing.

Five big pharma stocks claim the highest net margins in the industry: Gilead Sciences (NASDAQ:GILD), Amgen (NASDAQ:AMGN), Novartis (NYSE:NVS), Biogen (NASDAQ:BIIB), and Bristol-Myers Squibb (NYSE:BMY). Here's how these drugmakers generate loads of money. 

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1. Gilead Sciences

Gilead Sciences has the highest profit margin of any major drugmaker right now. The big biotech's revenue over the past 12 months totaled $29.1 billion, with earnings during the period of $12.6 billion. That translates to a net margin of 43.4%. 

Although Gilead doesn't break down profitability by product, the biotech's HIV drugs generated the most revenue for the company in the first quarter. Fast-growing Genvoya and longtime leader Truvada stood at the top of Gilead's HIV franchise in sales.

Gilead's hepatitis C franchise was the next most important contributor to the biotech's profitability. However, sales for hep C drugs Harvoni and Sovaldi are dropping significantly. As a result, Gilead's net margins are also falling.

2. Amgen

Amgen takes the No. 2 spot among big pharma stocks in terms of profitability. The biotech generated revenue of $22.9 billion and earnings of $7.9 billion over the past 12 months. This performance gives Amgen a net margin of 34.5%. 

Two drugs make roughly 46% of Amgen's total sales -- bone marrow stimulant Neulasta and autoimmune disease drug Enbrel. If current trends for these two drugs continue, Amgen's profitability could suffer. Enbrel's sales dropped 15% year over year in the first quarter, while sales for Neulasta increased only 2% compared to the prior-year period.

Amgen could potentially see net margins rise if cholesterol drug Repatha achieves the expectations that the company has for it. Many expected the drug to become a huge blockbuster for the biotech, but payers have been reluctant to pay for Repatha so far. 

3. Novartis

Novartis ranks just behind Amgen with a net margin of 34.2%. The Swiss drugmaker made revenue of $49.4 billion over the past 12 months, with earnings of nearly $6.4 billion.

Oncology drugs are a key driver of Novartis' profitability. Nine cancer drugs generate nearly one-quarter of the company's total revenue. Gleevec, which treats chronic myeloid leukemia and gastrointestinal stromal tumor, continues to be Novartis' top-selling cancer drug.

Cancer treatments

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Novartis also makes more than 20% of total sales from its Sandoz division, which develops and markets generic drugs, biosimilars, and anti-infectives. Its Alcon eye-care segment also contributes around 12% of total revenue. 

4. Biogen

Biogen reported revenue of $11.53 billion during the past 12 months. The biotech's earnings during the period totaled nearly $3.5 billion, which gives Biogen a net margin of 30.2%.

Its multiple sclerosis (MS) franchise continues to be the primary factor behind Biogen's profitability. However, that could be changing for a couple of reasons. One is that Biogen expects to benefit from growing sales of spinal muscular atrophy drug Spinraza. The drug, which Biogen licensed from Ionis Pharmaceuticals, could reach peak annual sales of $2.5 billion.

There is a potential dark cloud hovering over Biogen, though, which could negatively impact profitability down the road. Sales growth for the biotech's MS drugs has tapered off. Biogen faces the prospects of new rival MS drugs that could take away market share from its MS lineup.  

5. Bristol-Myers Squibb

Bristol-Myers Squibb (BMS) ranks as the fifth most profitable big pharma stock. BMS posted revenue of nearly $20 billion in the past 12 months, with earnings of $4.8 billion. This performance gave the company a solid net margin of 24.2%. 

Like Amgen, BMS has two drugs that stand out in importance to the company's profitability. Cancer drug Opdivo nearly 23% of total revenue in the first quarter of 2017. Anticoagulant Eliquis contributed 22% of total revenue during the period.

Although Opdivo's failure in a late-stage study targeting first-line lung cancer caused analysts to cut their forecasts for the drug, it should still become increasingly important for BMS' profitability in the future. Opdivo could eventually make more than $10 billion annually for Bristol-Myers Squibb.  

Keith Speights owns shares of Gilead Sciences. The Motley Fool owns shares of and recommends Biogen, Gilead Sciences, and Ionis Pharmaceuticals. The Motley Fool has a disclosure policy.