Biotech stocks offer us a wide range of investing opportunities. You could buy shares of a research stage company and bet on a potential game-changing product that may launch several years from now. Or you could invest in commercial stage companies that might be about to enter a new phase of growth. Right now, I'll focus on the latter and talk about biotech giant Vertex Pharmaceuticals (VRTX 0.77%) and smaller player Axsome Therapeutics (AXSM -1.44%).

Both of these companies are increasing revenue right now, and growth catalysts lie on the horizon. Vertex shares have climbed this year, but considering the biotech's prospects, the shares still have plenty of room to run. Axsome stock has slipped, offering us a particularly interesting buying opportunity. Let's take a closer look at each of these top stocks to buy in October.

1. Vertex Pharmaceuticals

Vertex Pharmaceuticals is the leader in the world of cystic fibrosis (CF) treatment, and it expects this dominance to last into at least the late 2030s. Today, Vertex sells blockbuster Trikafta, a product that last year brought in $7.6 billion in revenue, and other CF drugs.

Trikafta has the ability to treat about 90% of CF patients, but Vertex hasn't forgotten about the others. With Moderna, it's working on a candidate to address these particular patients and has started enrolling patients in a clinical trial for a candidate to treat them. Vertex also is studying a candidate in phase 3 trials that involves taking a single dose daily rather than the twice daily regimen of Trikafta. So, there's reason to be confident about ongoing growth from the CF program.

But another revenue driver may be right around the corner, and it will show Vertex has what it takes to expand beyond its specialty area. Vertex and partner CRISPR Therapeutics are awaiting a regulatory decision in December for exa-cel in the indication of sickle cell disease. Regulators will decide on the investigational treatment for beta thalassemia in March. If approved, Vertex will keep 60% of the profits from this potential blockbuster.

Vertex also is working on a pipeline of other interesting candidates, including a possible treatment for the common problem of pain. The company expects to wrap up the pivotal program later this year, so this candidate could represent another launch in the not-too-distant future.

Even though Vertex has advanced this year, the stock still looks reasonable at only 23 times forward earnings estimates considering its long-term prospects.

2. Axsome Therapeutics

Axsome Therapeutics started selling its first two drugs last year: Sleep disorder drug Sunosi, acquired from Jazz Pharmaceuticals, and antidepressant, Auvelity. So Axsome has just transitioned into the world of commercial stage companies, and so far, things are going pretty well.

Sunosi total prescriptions increased in the double digits in the most recent quarter year over year. And Auvelity saw a 72% increase in prescriptions from the first quarter of this year to reach a total of more than 53,000. After a strong Auvelity launch, the company decided to expand its sales force to broaden its reach from about 26,000 doctors to the 44,000 doctors responsible for more than 80% of antidepressant prescriptions.

All of this means we should expect growth in the coming months from these two products, offering potential catalysts for Axsome shares.

The long-term picture looks positive too, with Axsome predicting a portfolio of eight central nervous system products to generate as much as $11 billion in peak U.S. sales. This includes Sunosi and Auvelity as well as candidates Axsome aims to launch mainly between 2024 and 2025. These candidates all are in late-stage development -- this doesn't mean they'll make it to commercialization as failure can happen at any stage, but they have at least demonstrated a certain level of safety and efficacy in clinical trials.

Today, Axsome is trading at around 16 times sales, its lowest ever -- and at a time when we have a decent amount of visibility on future sales. This represents a solid opportunity for investors looking for an up-and-coming biotech player that could win in the near term as it ramps up product sales -- and over the long term too.