Bristol-Myers's Squibb (NYSE:BMY) and Pfizer's (NYSE:PFE) Eliquis, an anticoagulant challenger to both warfarin and Johnson & Johnson's (NYSE:JNJ) and Bayer's Xarelto, has seen its sales soar over the past six quarters.

Eliquis, a factor Xa inhibitor used by doctors to prevent blood clots in heart disease patients and patients following orthopedic surgery, is quickly gaining ground on Johnson & Johnson's Xarelto, a billion-dollar blockbuster the company rolled out in 2011. Since Eliquis sales are jumping and pose a potential threat to Johnson & Johnson's market share, let's take a closer look.

Source: Bristol-Myers Squibb.

First, a bit of background
Bristol and Pfizer's Eliquis is among a slate of next-generation anticoagulants that have won FDA approval and that vie for a share of a market long dominated by warfarin.

Despite warfarin's widespread use, doctors and patients are welcoming alternative treatments because warfarin requires regular monitoring and dose adjustments and is associated with brain hemorrhage.

Factor Xa drugs like Eliquis and Xarelto sidestep some of those drawbacks because they work differently from warfarin. Instead of inhibiting vitamin K's clotting ability, these drugs reduce thrombin production by inhibiting an enzyme called factor Xa.

As a result, they're quickly winning over doctors, and that's sending sales for Johnson & Johnson, Bristol-Myers, and Pfizer higher.

Source: Johnson & Johnson.

Comparing the two
Although Xarelto and Eliquis both target factor Xa, Xarelto was the first to gain FDA approval, and that's given it a valuable head start in terms of market share.

Both drugs are approved for use in elective knee and hip surgery or in prevention of stroke or systematic embolism with non-valvular atrial fibrillation. However, once-daily Xarelto was also approved for use in deep vein thrombosis and pulmonary embolism, giving it an advantage -- up until now.

Eliquis, which is taken twice daily, won the FDA's nod for use in those indications last month, and that expansion of the Eliquis label could mean that some of Xarelto's market share may be in jeopardy. 

If that proves true and Eliquis can carve away scripts from Xarelto, Bristol-Myers and Pfizer may find that Eliquis has the potential for billion-dollar sales, given that Xarelto racked up global revenue of $361 million in the second quarter, up 91% from last year. For comparison, Bristol-Myers reported that Eliquis sales totaled $171 million in the second quarter, up more than 300% from $41 million last year. 

Source: Bristol-Myers Squibb and author's calculations

Fool-worthy final thoughts
Investors are right to wonder whether Eliquis' quicker growth rate means that Xarelto will lose ground to it this year. However, before investors give up on Xarelto, they should recognize that both of these therapies may be able to coexist. 

The tallest hurdle facing factor Xa drugs such as Xarelto and Eliquis is the absence of an approved drug that can reverse their effects. That hurdle, however, may soon be overcome, since both companies are helping fund phase 3 trials of a factor Xa antidote being developed by Portola. If those trials play out as hoped, both Xarelto and Eliquis should become more appealing to doctors, sending sales higher for both.

Todd Campbell is long Portola. Todd owns E.B. Capital Markets, LLC. E.B. Capital's clients may or may not have positions in the companies mentioned. Todd owns Gundalow Advisors, LLC. Gundalow's clients do not have positions in the companies mentioned. The Motley Fool recommends and owns shares of Johnson & Johnson. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.