Want a good income-producing investment to add to your portfolio? There are many high-yielding stocks you can buy right now at attractive discounts. And in addition to generating a lot of dividend income for your portfolio, these investments also have the potential to produce significant gains for you in the long run.

Three absurdly cheap dividend stocks that are trading at modest multiples of their expected earnings today are Merck (NYSE: MRK), Gilead Sciences (NASDAQ: GILD), and Novartis (NYSE: NVS). If you're looking for stocks to buy and hold for years, here's why these can be excellent options to consider.

Merck

Pharmaceutical titan Merck pays investors a dividend that yields 2.8% -- more than double the S&P 500 average of 1.3%. And based on analyst expectations, it's also incredibly cheap, trading at a forward price-to-earnings multiple (P/E) of less than 11.

Investors are discounting the stock a bit due to its heavy exposure to cancer-fighting drug Keytruda; it faces a patent cliff later this decade that has investors worried about Merck's growth prospects.

But the company has been innovating, and regulators recently approved Winrevair for pulmonary arterial hypertension. At its peak, the drug could generate over $6 billion in revenue for Merck. The company also expects that its cardiovascular portfolio alone may bring in $10 billion by the end of the decade.

While another drug like Keytruda (which generates more than $20 billion in annual revenue) might not be in Merck's portfolio today, by investing in more growth opportunities and expanding its pipeline, the business remains on a positive path forward.

There's some risk, as is always the case with large patent cliffs, but Merck still has time via acquisitions and pipeline expansion to bolster its prospects. And Keytruda's revenue isn't going to suddenly go straight to zero once its patents expire, either.

For long-term investors, Merck makes for an attractive investment to load up on today.

Gilead Sciences

Investors can secure an even higher-yielding stock with Gilead Sciences. It currently pays 3.6%, and this is with the stock rallying in recent months and now trading near its 52-week high. Prior to that, the yield was even higher.

Gilead is a top HIV treatment company, and its focus on that area of healthcare ensures it has a stable and steady business to build around. HIV remains a big concern in healthcare, but Gilead could play a significant role in helping to prevent infections.

In a recent trial, its twice-yearly injectable treatment, lenacapavir, demonstrated 100% efficacy. The company says it was the first phase 3 trial for HIV prevention where there were no infections.