The longer you're willing to hold a high-quality dividend stock, the more years of passive income your portfolio will enjoy. But as wise investors know, safe investments aren't always easy to find, and there are quite a few risky stocks that probably won't be able to keep paying their dividends for as long as you might prefer. 

With that in mind, there are at least two healthcare businesses that are almost certainly going to make good on their promises of passive income over the coming decades. You might not beat the market with an investment in either, but if you're content to collect cash and sleep calmly at night no matter what's going on in the economy, they'll both be right up your alley. 

1. Pfizer

Besides Pfizer's (PFE 0.23%) recent and highly successful launch of the Comirnaty coronavirus vaccine, its safety as an investment stems from its huge portfolio of medicines for sale, which itself is fueled by a steady stream of new drugs hitting the market in any given year.

The new medicines that it expects to commercialize within the next two years are anticipated to yield as much as $20 billion in annual revenue by 2030. And 11 of those newcomers could be launched in 2023 alone, pending regulatory approval.

Considering the company's trailing-12-month revenue is around $99.9 billion, the anticipated sum of fresh revenue in 2030 might not seem like much growth. And it's entirely possible that between now and then, the top line will shrink, as it faces perpetual challenges from lapsing exclusivity protections on its drugs that allow generic competitors to steal market share.

But there's often a trade-off between growth potential and stability, and Pfizer is very much a stock that skews toward stability even in light of its ongoing battle for market share.

At the same time, it has been attentive to its dividend, which it has increased 71% over the past decade. And with a payout ratio that's a scant 30% of earnings, management can probably continue to increase the dividend sustainably. If you buy Pfizer stock and hold it forever, you'll likely reap the rewards of the successes in the company's pipeline over time and benefit from its ongoing share repurchases, too.

2. Becton, Dickinson

You might not have ever heard of Becton, Dickinson (BDX 0.20%), but if you've ever received medical care, there's a very strong chance that you've contributed to demand for some of its massive selection of medical goods.

Besides making boring-yet-indispensable products like syringes, test tubes, and catheter sets, it also makes high-tech equipment like anesthesia workstations and analyzer devices for biomedical research laboratories. In total, it brought in more than $4.7 billion in the fourth quarter alone, roughly half of which was derived from its medical products segment.

Demand for this crucial medical hardware isn't going to drop anytime soon, nor is the demand for its more esoteric offerings of medical disposables that are necessary to provide hospital care.

And while the company does need to perpetually spend some money on developing new products, it isn't about to get into an innovation arms race with a competitor for many of the basic medical or research goods it sells as there are only so many additional features something like a test tube can have.

Furthermore, such a mix of disposable and durable goods means that it brings in plenty of recurring revenue each quarter, which in turn gives it the steady cash flow it uses to pay its dividend for years and years.

For each of the last 51 years and running, management has raised the dividend, making the company a Dividend King. And since the end of 2013, its payout has grown by 84%, whereas its quarterly FCF has risen by an impressive 373%. So it looks like there's more than enough growth to justify continuing with dividend hikes.

As long as Becton, Dickinson keeps developing and manufacturing products that the medical system needs to deliver care, it'll be a lucrative stock to hold for the coming years.