Thomas McCourt
Analyst · JPMorgan
Thanks, Meredith. Good morning, everyone, and thanks for joining us today. We had a solid start to 2021, as demonstrated by strong LINZESS performance, continuing its momentum from 2020. And another quarter of delivering profitability and positive cash flow. We are steadfast in our mission and importantly, remain focused on our goals of driving value to our shareholders by bringing important medicines to patients and building a growing and successful business.
Our strategy starts with maximizing LINZESS. LINZESS continued to demonstrate double-digit growth in demand in the first quarter, resulting in U.S. net sales of $215 million or 12% growth year-over-year, and we believe there's substantial opportunity for continued growth moving forward. Second, we're seeking to build an innovative portfolio both through the development of internal assets such as our own IW-3300 for the potential treatment of visceral pain conditions and through bringing in external assets that target serious, organic GI diseases.
And finally, we're focused on generating sustainable cash profits and cash flow. We ended the first quarter with nearly $440 million in cash on our balance sheet, an increase of $75 million versus the fourth quarter in 2020. Our strong financial position and our performance to date provides us with the ability to simultaneously invest thoughtfully in our strategic priorities, as we just highlighted, and be in a position to also return cash to shareholders. We announced this morning that our Board has authorized a share repurchase program under which we may purchase up to $150 million in outstanding shares of Ironwood common stock through December 2022.
We maintain a disciplined and thoughtful approach to capital allocation and are committed to strategically deploying capital where we believe it can drive the greatest value to our shareholders. The share repurchase program is aligned with that commitment and underscores the strength of our financial position.
Now let's turn to the commercial performance. LINZESS performed remarkably well in the first quarter. Prescription demand grew 12% year-over-year, with LINZESS continuing to strengthen its position as the #1 prescription treatment in the U.S. for adults with IBS-C and chronic constipation, with approximately 40% market share in the category. New-to-brand prescriptions increased 24% year-over-year, significantly outpacing the IBS-C and chronic constipation market. This is an all-time high for LINZESS, reinforcing that patients and health care practitioners continue to choose LINZESS. 90-day prescriptions also continue to represent a significant number of total LINZESS prescriptions.
In the first quarter, 90-day prescriptions made up nearly 20% of the business. This strong performance reaffirms our confidence that LINZESS has the potential to exceed $1 billion in U.S. net sales with several additional opportunities to drive even further growth. The success of LINZESS to date in the U.S. has come through consistent and strong execution of the commercial strategy that's grounded in 3 core fundamentals: increasing awareness of LINZESS amongst physicians, motivating appropriate patients to seek care and request LINZESS and securing broad payer access. We believe the growth that we've achieved in the first quarter is in part a reflection of the success of this strategy. And the positive experience that physicians have had with LINZESS.
In addition, LINZESS remains the class leader in formulary access, important for supporting both existing and new patients. We also believe that the brand continues to demonstrate strong promotional responsiveness. Since the beginning of the year, we've evolved the marketing mix to further support our consumer media strategies. This includes a fully refreshed consumer campaign that launched in early April, which now incorporates the IBS-C overall abdominal symptom data. We expect this new campaign to have a meaningful impact on patients and the brand. Also, our skilled and experienced salesforce have been gaining a high level of in-person meetings with physicians, achieving access comparable to pre-COVID levels.
Additionally, there has been significant increase in online searches and clicks for constipation-related terms. And we are also seeing higher traffic to the LINZESS website, suggesting that more patients are seeking treatment options for their IBS-C and constipation-related symptoms. In the first quarter, we experienced a significant increase in the number of visitors to linzess.com with about 400,000 unique visitors a month. This represents a 70% increase year-over-year. Finally, recent survey data also suggests that there's been a significant increase in overall prevalence of IBS-C and chronic constipations in adults in the U.S.
New data on prevalence and the burden of illness will be presented at DDW in a few weeks. These data illustrate the growth potential for LINZESS for appropriate patients particularly on the relief of overall abdominal symptoms associated with IBS-C. We're looking forward to the upcoming DDW meeting, where Ironwood and AbbVie will be presenting 7 scientific posters, 4 of which have been developed in partnership with AbbVie, and 4 of the 7 have been named posters of distinction.
In addition to the demand catalysts I highlighted, we continue to believe that our investments in telemedicine may help advance access to physician care for GI patients in need and our commitment to exploring life cycle management initiatives present an opportunity to expand the clinical utility for linaclotide. We believe both these efforts will serve as key drivers for continued growth moving forward.
Just a few final comments before I turn it over to Gina. As we look forward, we recognize that GI remains a therapeutic area with a significant unmet need with millions of highly symptomatic and suffering patients and diseases that often have limited or no treatment option. We have a strong experienced team with the knowledge and expertise to advance innovative science and build successful brands. I'd like to take a moment to thank all of our employees, patients, caregivers, and advocates in the GI community for their shared dedication to advancing and supporting our efforts in this underserved market.
With that, I'll turn the call over to Gina.