Lilly's 1923 launch of Iletin began a century of diabetes drugs

Eli Lilly and Company introduced Iletin in 1923, which the company describes as the world's first commercially available insulin product, for a disease that was then fatal and had no effective treatment options ^[2]. The product was an animal-source insulin ^[2].

Lilly's corporate history page shows before-and-after photographs of a patient treated with what it calls the world's first commercially available insulin product ^[2]. Separately, Wikipedia lists Lilly as the first company to mass-produce both the polio vaccine and insulin ^[1]. The two accounts use different wording for the same claim: Lilly calls Iletin the first commercially available insulin product ^[2], while Wikipedia describes the company as the first to mass-produce insulin ^[1]. Neither account describes how the insulin was developed, who worked on it, what it cost, or how many patients received it. Those details have not been reported in these sources.

Lilly was founded in 1876 by Colonel Eli Lilly, a pharmaceutical chemist and Union army veteran ^[1]. Insulin remained central to the company's work. In 1982, Lilly introduced Humulin, human insulin derived from rDNA, which the company describes as the world's first human-health-care product created using recombinant DNA technology ^[2]. Wikipedia also lists Lilly as an early producer of human insulin through recombinant DNA, including Humulin, insulin lispro (Humalog), and Basaglar (insulin glargine), which it describes as the first approved biosimilar insulin product in the United States ^[1]. Humulin and Humalog now appear among the company's products with a minor contribution to revenues ^[1].

The same line of work later produced the incretin drugs that drive Lilly's business now. In 2005 the company brought exenatide to market, the first of the GLP-1 receptor agonists ^[1]. Tirzepatide followed as a GIP and GLP-1 receptor agonist, sold as Mounjaro and Zepbound ^[1]. Mounjaro was introduced in 2022 to treat diabetes ^[2], Zepbound was approved in 2023 for chronic weight management ^[2], and in 2024 it was approved as the first prescription medicine for adults with moderate-to-severe obstructive sleep apnea and obesity ^[2].

The financial scale has shifted with it. Tirzepatide accounted for 56% of Lilly's 2025 revenues, while the older diabetes drug dulaglutide (Trulicity) accounted for 7% ^[1]. Lilly reported $65.18 billion in revenue for 2025 ^[1]. In November 2025 the company reached a $1 trillion market capitalization, which Wikipedia describes as the first health-care company to do so ^[1].

Lilly's published history also lists a series of patient-facing insulin programs: a camp program launched in 2001 that the company says donated more than $25 million in insulin, supplies, scholarships and educational resources; a diabetes call center opened in 2018; a $35 copay card launched in 2020; and an expansion of that program in 2023 ^[2].

Why it matters for patients

The 1923 launch is history, not a new product or approval, but it explains the shape of the market patients deal with now. The company that makes tirzepatide built its first major franchise on insulin ^[2], and insulin products such as Humulin and Humalog are now listed among those with a minor revenue contribution, while tirzepatide supplies most of Lilly's revenue ^[1].

For people who take insulin, the practical detail in these accounts is that Lilly's own history describes discount and support programs, including a $35 copay card launched in 2020 and expanded in 2023 ^[2]. Whether those programs apply to any individual depends on the medicine and insurance situation, and the sources do not describe eligibility rules or current enrollment numbers.

For people taking or considering tirzepatide, the historical link is indirect. The sources do not say whether the insulin business funded or shaped the later incretin programs, and that connection has not been disclosed. What the sources do show is concentration: one molecule, tirzepatide, produced 56% of Lilly's 2025 revenue ^[1].

What is not known

The sources do not state how Iletin was priced in 1923, how it was manufactured, how many patients used it, or what role any outside partner played in its development. No current U.S. list price, insurance coverage detail, or supply information for Iletin appears in these accounts, and Iletin is not listed among the company's current products ^[1][2].