Insulin glargine-yfgn is a long-acting insulin sold under the brand name Semglee. It is a biosimilar to Lantus (the original insulin glargine) and is used to manage blood sugar in adults and children with type 1 or type 2 diabetes. What makes it notable is its designation as an interchangeable biosimilar, meaning pharmacists in most cases can substitute it for Lantus without needing separate approval from the prescriber.
What the “-yfgn” Suffix Means
The four-letter suffix attached to the name is an FDA naming convention applied to all biological products, including biosimilars. The suffix “yfgn” has no inherent meaning. It exists purely to distinguish this specific product from other versions of insulin glargine in pharmacy records, adverse event reports, and insurance claims. This helps healthcare professionals and patients track exactly which product was dispensed, which matters for safety monitoring. Every biosimilar and originator biologic gets its own unique suffix.
How It Differs From Lantus
In clinical terms, it doesn’t. Insulin glargine-yfgn works the same way as Lantus: it starts working about 2 hours after injection, has no pronounced peak, and provides steady blood sugar control for up to 24 hours. Clinical trials showed virtually identical reductions in HbA1c (a measure of average blood sugar over 2 to 3 months) between insulin glargine and its comparators across studies in type 1 diabetes, type 2 diabetes, and pediatric patients.
The real difference is cost and access. A box of long-acting insulin pens from major brands typically runs $150 to $500 depending on the brand and pharmacy. One version of insulin glargine-yfgn, manufactured by Civica, is available at a consumer-facing price capped at $55 per box of five pens, making it the lowest list price in the current long-acting insulin market.
What “Interchangeable Biosimilar” Means for You
Biosimilars are biologic medications that are highly similar to an already-approved reference product with no clinically meaningful differences. An interchangeable biosimilar goes one step further: it has met additional FDA standards that allow pharmacists to swap it in place of the reference product (Lantus) at the pharmacy counter. In practice, this means your pharmacist can fill a Lantus prescription with Semglee without calling your doctor first, similar to how generic drugs are substituted for brand-name medications.
State laws do vary on the details. Some states require pharmacists to notify both the prescriber and the patient about the switch. In Pennsylvania, for example, pharmacists must inform the patient at the time of dispensing (including any price difference) and notify the prescriber within 72 hours. Prescribers can also explicitly block substitution if they choose. But the default in most states is that the switch is permitted.
How It’s Used
Insulin glargine-yfgn is injected under the skin once daily, typically at the same time each day. It comes in two forms: a 10 mL multiple-dose vial and a 3 mL prefilled pen. Both contain 100 units per mL (U-100). It is often paired with a rapid-acting or short-acting insulin to cover blood sugar spikes around meals, since it provides a steady baseline level of insulin rather than a burst to handle food.
It is not used to treat diabetic ketoacidosis, a serious complication that requires fast-acting insulin and emergency care.
Side Effects
Low blood sugar (hypoglycemia) is the most common side effect, which is true of all insulin products. In clinical studies of adults with type 1 diabetes, severe hypoglycemia occurred in roughly 7% to 11% of patients using insulin glargine over study periods of 16 to 28 weeks. Rates were comparable to those seen with NPH insulin, an older type of long-acting insulin. In children with type 1 diabetes, severe hypoglycemia rates were higher in both treatment groups, reaching about 23% over 26 weeks.
For adults with type 2 diabetes, severe hypoglycemia was much less common, ranging from less than 1% to about 8% depending on the study and its length. A large long-term study with a median follow-up of 6.2 years found severe hypoglycemia in 5.6% of insulin glargine users compared to 1.8% receiving standard care alone.
Injection site reactions are another possibility, including redness, pain, itching, swelling, and inflammation. In clinical trials, injection site pain was reported in 2.7% of insulin glargine users versus 0.7% on NPH insulin, though none of these cases led patients to stop treatment. Other reported side effects include allergic reactions, changes in fat tissue at injection sites (lipodystrophy), rash, swelling, and weight gain.
Storage
Unopened vials and pens should be stored in a refrigerator (36°F to 46°F). Once you start using a vial or pen, it can be kept at room temperature (below 86°F) for up to 28 days. After 28 days at room temperature, any remaining insulin should be discarded. The solution should always look clear and colorless. If it appears cloudy or contains particles, do not use it.