Some stocks have the "it" factor. They've got such tremendous growth prospects that investors want to add them to their portfolios without any hesitation.

Three Motley Fool investors think they've found three such unstoppable stocks to buy sooner rather than later. Here's why they like Eli Lilly (LLY 0.13%), Novo Nordisk (NVO 1.79%), and Vertex Pharmaceuticals (VRTX 0.47%).

Eli Lilly's business still hasn't taken off yet

David Jagielski (Eli Lilly): Although it's up around 70% this year, Eli Lilly is a stock that's still nowhere near its peak. The business has some incredible drugs in its portfolio that could lead to tens of billions of dollars in revenue in the years ahead.

Mounjaro, its diabetes treatment, is already generating revenue. But the real potential is if the drug obtains approval from the U.S. Food and Drug Administration (FDA) to treat obesity: That could open up the floodgates to billions more in revenue.

Analysts are having a hard time estimating just how much growth there could be for Mounjaro, as it could help treat many obesity-related illnesses. Some estimates have pegged it at $50 billion, others at $68 billion, and even one at $100 billion in annual sales. Many of the top healthcare companies in the world don't generate that much in revenue -- and Mounjaro alone may accomplish that feat.

On top of that, Eli Lilly also has donanemab, a treatment for Alzheimer's disease. In clinical trials, donanemab was similar in effectiveness to Biogen's Leqembi, which has already received regulatory approval. Even if donanemab fights for market share alongside Leqembi, that's another multibillion-dollar opportunity for Eli Lilly.

The healthcare stock does trade at an incredibly high 85 times trailing earnings, which may spook some investors. But with the opportunities for Mounjaro and donanemab along with close to two dozen phase 3 programs in the pipeline, there's plenty of growth on the horizon for Eli Lilly's business. Given the potential ahead, the stock still has much more upside, making it a great buy right now.

Novo Nordisk is showing no signs of slowing down

Prosper Junior Bakiny (Novo Nordisk): It's been a great past 12 months for Novo Nordisk and its shareholders, as the stock has been crushing the market. The drugmaker has made headlines thanks to its portfolio of products that treat diabetes and obesity. Novo Nordisk has even dealt with a well-documented shortage of its weight-loss treatment Wegovy and has sought to ramp up manufacturing to meet the soaring demand.

On the one hand, those who have been paying attention to Novo Nordisk for a long time won't be too surprised. It's been a leader in this area for a while. Whether with insulin products or other therapies designed to help diabetes patients control their blood sugar levels or their weight, hardly any company has done a better job at producing new medicines along those lines.

That's also one of the best reasons to buy Novo Nordisk's shares. Beyond its strong financial results and excellent stock performance over the past year, its almost unmatched expertise in this area means it can develop more blockbuster medicines. And there will be a greater need for them. Diabetes has been on the rise for decades, and has turned into a global health crisis that causes considerable strain on healthcare systems.

No one would celebrate this trend, but Novo Nordisk's position in this market is why it can continue performing well, although perhaps not always as well as it did recently. But in five years, the biotech will almost certainly be substantially up from its current levels. Don't wait too long before getting in on the action.

Vertex is on the path to becoming even more unstoppable

Keith Speights (Vertex Pharmaceuticals): Arguably the best way to truly be unstoppable is to not have any competition. Vertex Pharmaceuticals checks off that box. It markets the only drugs that treat the underlying cause of the rare genetic disease cystic fibrosis (CF).

Vertex could soon become even more unstoppable in CF. The company expects to wrap up late-stage testing of its vanzacaftor triple-drug combo by the end of the year. This could be Vertex's best CF therapy yet, as it requires only a once-daily dosing. It's also, thanks to lower royalty payments, likely to be more profitable than the company's other CF drugs.

CF isn't Vertex's only opportunity, though. The FDA has established a PDUFA date of Dec. 8 this year for an approval decision on exa-cel in treating sickle cell disease. A second decision on approval in treating transfusion-dependent beta-thalassemia will be on the way by March 30, 2024.

VX-548 is poised to be another big winner for Vertex outside of CF. The non-opioid pain drug is currently in late-stage studies that are on track to be completed by year-end. Millions of people in the U.S. suffer from acute pain. VX-548 could offer alternative pain relief without the addictive potential and side effects associated with opioids.

In addition, Vertex continues to advance inaxaplin in a pivotal clinical study targeting APOL1-mediated kidney disease (AMKD). CF affects around 88,000 patients worldwide, but around 100,000 patients have AMKD.

Like Eli Lilly, Vertex hasn't fully taken off yet despite its robust sales growth in CF. Like Novo Nordisk, the company shows no signs of slowing down thanks to its promising pipeline.