It's a fact: Dividend stocks are often the foundation of most retirement portfolios. But not all dividend stocks are created equally. Income investors are usually on the lookout for stocks with as high a dividend yield as possible, but also a yield that'll be sustainable over time. Finding companies that can balance a healthy dividend yield with future growth potential are few and far between.

Though healthcare is not often an industry that income investors flock to, the one exception would be Big Pharma drug stocks. Of the slightly more than one dozen large drugmakers (think $50 billion market cap, or larger) lumped in as "Big Pharma," every single one pays a dividend to investors. And since drugmakers hold a bounty of pricing advantages in the U.S., their profitability and cash flow are rarely in question. In essence, Big Pharma stocks that are in a bit of a growth lull and that have seen their share prices slump could be perfect targets for income investors.

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If you're looking to sit back and collect 4% or more per year in dividend yield, then here are three Big Pharma stocks with top-tier dividends to consider.

GlaxoSmithKline

One Big Pharma stock that could make the long-term wait tolerable is U.K.-based GlaxoSmithKline (NYSE:GSK). Over the trailing two-year period, shares of Glaxo are down by nearly 5%, while the broad-based S&P 500 has risen by 14%.

Why no love for this drug giant from across the pond? The biggest issue has been the expected decline of COPD and asthma blockbuster drug Advair. GlaxoSmithKline's lead drug, which at one time generated in excess of $8 billion in annual sales but lost its patent protection years ago, recently had a generic version of the drug hit pharmacy shelves. The expectation of lost sales has mostly held GSK, as GlaxoSmithKline is also known, back.

However, the company's burgeoning product portfolio, its corporate transformation, and its dividend, may make it a worthwhile income stock to consider.

To begin with, GSK is generating more from the sale of new drugs than it's losing from the generic competition of mature therapies. That's really great news for long-term investors worried about Glaxo's future growth prospects. The company's long-lasting new COPD and asthma medications are beginning to catch on with insurers and physicians, and its HIV therapies Triumeq and Tivicay and blazing hot. In the first quarter, Triumeq and Tivicay grew constant currency sales by 45% and 41%, respectively, and combined for $1.08 billion in sales.

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GSK is also benefiting from a 2015 reorganization that saw it sell its oncology portfolio to Novartis (NYSE:NVS) for $16 billion, and acquire Novartis' vaccine portfolio (minus influenza) for about $7 billion. As you can see, this restructuring also netted GSK a nice sum of cash. The two companies also formed a joint-venture for their consumer health products that saved money by reducing redundancies and improved pricing power.

Finally, GlaxoSmithKline has stuck by its dividend payout, and is currently yielding 4.6%. Glaxo's management team has suggested it's payout is sustaining, and its growing new drug sales suggest they're correct. Despite underperforming the broader market, that's a tidy sum to collect while you wait for its respiratory and HIV segments to really thrive.

Pfizer

The story is very much the same for Pfizer (NYSE:PFE), which has seen its share price dip by nearly 7% over the trailing two-year period, putting it almost 21-percentage-points behind the broader market index over that timeframe.

But unlike GSK, Pfizer's patent issues tie into a number of drugs, not just one. It certainly hurt when cholesterol-lowering drug Lipitor went off patent earlier this decade. Lipitor may have been dethroned by Humira in terms of highest annual sales, but it's still the all-time best-selling drug. Of course, its sales have fallen from more than $13 billion annually to around $1.8 billion in 2016. Other blockbuster drugs that have been hit hard by generic competition include Celebrex, Zoloft, Xanax, and Zyvox, to name a few.

The good news is Pfizer has a number of Food and Drug Administration-approved drugs that could help fill the void over time, as well as a pipeline of potentially intriguing candidates.

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For example, advanced HER2- breast cancer drug Ibrance is expected to be a major growth driver over the next decade. In March, Pfizer announced that the FDA had approved a supplemental new drug application for Ibrance expanding its use to advanced first-line HR+, HER2- breast cancer. The press release notes that more than 50,000 patients have been prescribed Ibrance since it was first approved, and first-quarter sales growth of 59% to $679 million demonstrates what a force this cancer drug can be for patients, Pfizer, and its shareholders.

In terms of Pfizer's pipeline, the excitement tends to revolve around cancer immunotherapy Bavencio, which became the first checkpoint inhibitor approved to treat merkel cell carcinoma in March. The drug, which was developed in collaboration with Merck KGaA, also got the thumbs-up from the FDA to treat advanced bladder cancer in early May. With immunotherapies looking as if they could be the foundation of cancer treatments, especially combination therapies, for years to come, Pfizer has found new life as a cancer drug developer.

What do income investors get while they wait for Bavencio to blossom and Ibrance to expand? How about a 4% dividend yield and a healthy share repurchase program. Considering that Pfizer's payout ratio is only about 50% of this year's estimated EPS, its payout appears sustainable over the long run.

AstraZeneca

Last, but certainly not least, the most bountiful dividend among Big Pharma stocks, AstraZeneca (NYSE:AZN). This U.K. drug behemoth has, like its peers above, underperformed the broader market. Its shares are down around 2% over the past two years, underperforming the S&P 500 by almost 16 percentage points.

As is the case with Pfizer and GSK, AstraZeneca's underperformance can be traced to its patent losses. In recent years, blockbusters Nexium, Symbicort, and Crestor have all lost patent protection, paving the way for generic drugmakers to step in and seize market share. For example, Crestor sales plunged 44% on a constant currency basis during the first quarter to $631 million from the prior-year period. With sales in AstraZeneca's legacy oncology division also somewhat sluggish, it's taken a backseat to other Big Pharma stocks.

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However, glimmers of hope are beginning to flash before the eyes of Wall Street and investors. The approval of Tagrisso, a cancer drug that targets the T790M mutation in patients with non-small cell lung cancer (NSCLC) in 2015, has rejuvenated oncology growth for AstraZeneca. Sales of the drug rose from practically nothing in Q1 2016 to $172 million in the latest quarter. Furthermore, ongoing studies of Tagrisso could offer label expansion opportunities that make it a clear second-line treatment option for T790M mutation-positive NSCLC patients.

AstraZeneca's pipeline is also intriguing. The company is developing its own cancer immunotherapy drug known as durvalumab, which in many cases is also being paired with tremelimumab, another immune-boosting agent. Earlier in May, durvalumab got its first green light from the FDA as a treatment for advanced bladder cancer under the brand name Imfinzi. Back in 2014, AstraZeneca's management team suggested that durvalumab could have peak sales potential, including combinations, of up to $6.5 billion.

While investors wait for AstraZeneca's oncology portfolio to play out, they get to collect an extremely healthy 5.6% yield. If reinvested, AstraZeneca's yield alone would double your investment in under 13 years! But is it sustainable? That remains to be seen. At the moment AstraZeneca is on track to pay out almost all of its earnings as a dividend this year, which could be a warning that a cut could be coming. Strength in its oncology sales over the next couple of quarters will likely give us our answer. 

Sean Williams has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. The Motley Fool has a disclosure policy.