Do you just want to set it and forget it, so to speak? While this mindset is the essence of a buy-and-hold strategy, it also arguably oversimplifies it. Buying and holding is clearly a long-term thing, but that doesn't mean you should never check in on those stocks.

Some publicly traded companies out there, however, are so reliable regardless of the economic backdrop that they can be considered "unstoppable" for long-term investors. Here's a closer look at three of them you might want to consider adding to your portfolio while they're cheaper than they were just a few months ago.

1. Merck

Pharmaceutical giant Merck (MRK -0.65%) never really seemed like it was wholeheartedly working on a COVID-19 vaccine and seemed happy to let rivals like Pfizer and Moderna take up the mantle. And what little effort Merck had made on the coronavirus vaccine front was largely abandoned early last year when the company discontinued trials of its vaccine candidates V590 and V591. The reason? Lackluster effectiveness. Given all of this, it's not surprising Merck was one of the few drug stocks that underperformed for the better part of the past couple of years.

In retrospect, it's probably been more of a benefit than a liability that Merck didn't wade waist-deep into COVID vaccine waters.

A person reviewing stock charts on a laptop.

Image source: Getty Images.

Although the pandemic proved lucrative for some pharma names, that revenue surge was never going to be fully sustained. A great deal of other research and development was put on hold to focus on the creation of COVID vaccines. Now many of these companies' pipelines are a year or more behind.

But not Merck. It has 77 unique phase 2 trials underway and 29 phase 3 programs. That's in addition to three drug approval requests being reviewed by regulators right now. Its pipeline also consists of several new uses of cancer-fighting miracle drug Keytruda, which after its first approval back in 2018 has expanded into a franchise worth $17 billion per year. There's still more growth in the cards, though.

The market's starting to figure all of this out, by the way. After nearly two years' worth of tepid performance, Merck shares are now up more than 20% just since the end of 2021. However, there's more gas left in the tank for this pharmaceutical outfit that remained focused on its particular bigger picture.

2. Procter & Gamble

The Procter & Gamble Company (PG 0.47%) is the name behind many of consumers' most beloved brands, like Pampers diapers, Tide laundry detergent, and Bounty paper towels. Loyalty to these brands is strong, to be sure, yet it's not the only reason -- or even the best reason -- to own a piece of P&G.

Neither is its ability to pass along its higher costs to consumers. Although shoppers are grumbling about the higher prices they see while walking down stores' aisles, they're still paying those prices. Last quarter's organic sales for Procter were up 10% year over year, more or less in stride with its 16% uptick in its costs for the goods it sold during the three-month stretch.

Rather, the biggest reason Procter & Gamble is so unstoppable (and the reason you'll want to own this particular consumer goods stock) is its sheer size and its subsequent marketing budget. At $76 billion in revenue per year, this company is nearly twice as big as any of its direct competitors and can promote its brands more aggressively than any of its rivals. Depending on the year, there's a good chance P&G is the world's single-biggest advertiser in terms of total dollars spent on the effort. Don't dismiss the importance of this kind of promotional firepower.

3. Microsoft

Finally, add Microsoft (MSFT 0.06%) to your list of unstoppable stocks that would be welcome additions to nearly anyone's portfolio.

With just a quick glance at the stock's chart, it would be easy to question the bullish argument. Like so many of its technology peers, Microsoft shares are down more than 20% from their November high and seemingly still moving lower. Also, given its sheer age and size, certainly some investors are chalking the company up as a computer-centric has-been that's been eased into irrelevancy by newer, hipper technologies like mobile phones.

If that's what you're thinking, though, take a closer look at the Microsoft the CEO Satya Nadella has shaped since taking the helm a little over eight years ago. Cloud computing is a huge part of its business for corporate data centers as well as consumers that want browser-based access to apps like Microsoft Word and Excel. Not only do these offerings mean the company is as relevant today as it was 25 years ago when the PC boom was just taking root, but it also means the software giant enjoys a great deal of recurring revenue. In the meantime, Microsoft is forever expanding its video-gaming arm and digital advertising, furthering its push beyond the Windows operating system.

And it's hardly a slow-growth collection of business ventures. Analysts are calling for revenue growth of more than 18% this year and more than 14% next year, mostly because the company has evolved with the ever-changing technology arena.

Look for more of the same down the road, too. Until the world is willing to abandon the use of smartphones, game consoles, the cloud, and the internet, Microsoft has plenty of opportunities to plug into.