Insulin Prescribing Best Practices

The 30-second version

Basal of choice: pick the interchangeable glargine — insulin glargine-yfgn (Semglee). The pharmacy can swap it for whatever glargine the plan actually covers.
Leave DAW / "dispense as written" unchecked. Checking it defeats the whole point.
Write the full sig: units, route, frequency, timing — plus indication.
Send the pen needles or syringes on a separate Rx, and test strips + lancets if you want a titration log.
Size quantity to the in-use expiry, not just the units — low-dose patients discard half-full pens. Use the Insulin Quantity calculator.

Glargine: Lantus vs Basaglar vs Semglee vs Rezvoglar

All are insulin glargine 100 units/mL, same amino acid sequence, comparable PK/PD, and no clinically meaningful difference in A1c, hypoglycemia, or immunogenicity. Conversion between them is 1:1. The only difference that matters at the keyboard is FDA regulatory status, because that decides whether the pharmacist is allowed to substitute.

Product	FDA status	Pharmacy substitution
Lantus (Sanofi)	Reference biologic	Is the reference — an interchangeable may be swapped in for it
Semglee — glargine-yfgn (Viatris/Biocon)	Interchangeable biosimilar	Yes — pharmacist may substitute without calling you (per state pharmacy law)
Rezvoglar — glargine-aglr (Lilly)	Interchangeable biosimilar	Yes — same as above; the fallback if a plan prefers Lilly
Basaglar (Lilly)	Follow-on biologic (not interchangeable)	No — needs a brand-new Rx from you

The EHR order-entry trap

Searching glargine in most EHR order-entry screens and taking the first hit usually transmits Lantus — the reference brand — which on many formularies now sits on a non-preferred tier and kicks off a prior auth. "Write generically" is not enough when the EHR resolves generic to a brand.

Search the suffixed name — insulin glargine-yfgn — and select that product explicitly.
Confirm the transmitted line says glargine-yfgn / Semglee before signing.
Uncheck DAW so the pharmacy may substitute to the covered glargine.
If a plan hard-prefers Basaglar, no substitution is possible — you must send a new Rx.

Substitution authority for interchangeables is set by state pharmacy law. Louisiana's rule is below; check your own state before promising a patient the swap happens automatically.

Louisiana substitution law

FDA interchangeability is the federal trigger; state law decides what the pharmacist may then do. Semglee was approved as the first interchangeable biosimilar in July 2021, which is what activates the Louisiana provisions below.

Substitution is authorized. La. R.S. 37:1164(18)(b)(i) defines "equivalent drug product" to include a biological rated interchangeable in the FDA Purple Book — so an interchangeable biosimilar is legally an equivalent of the reference product. Framework established by HB 319 (2015), effective 8/1/2015.
You can block it. La. Admin. Code tit. 46, §LIII-2517(B)(1): no interchange when the prescriber prohibits it — handwritten DAW, a brand name given verbally, or DAW / "Brand Medically Necessary" electronically.
The patient must consent. §LIII-2517(B)(3) permits the interchange only if the patient has been informed of and consented to it. Worth a sentence at the visit — "the pharmacy may fill this as Semglee or another glargine; it's the same insulin" — so the swap isn't a surprise at the counter.
You get told what was dispensed. §LIII-2517(B)(4): the pharmacist must communicate the product name and manufacturer to you within five business days. Reconcile the med list against that, since the chart will still say what you sent.
No notice required when you prohibited interchange, when nothing is rated interchangeable/therapeutically equivalent, or on a refill unchanged from the prior fill — so silence does not mean the brand was dispensed.

Rule promulgated under La. R.S. 37:1182; §LIII-2517 amended LR 43:2162 (11/1/2017) and LR 46:793 (6/1/2020). Congress is considering the Biosimilar Red Tape Elimination Act, which would deem biosimilars interchangeable on licensure and collapse this distinction — not law as of July 2026.

Why this is worth the extra clicks

Biosimilar glargine list price is well below Lantus; real-world spending fell from roughly $191 to $147 per patient per month after biosimilar entry, with no change in outcomes.
Interchangeable designation drove a large jump in Semglee's formulary placement and dispensing — most commercial and Medicaid plans now cover an interchangeable glargine on a preferred tier.
Repeated switching between Semglee and Lantus (INSTRIDE 3) showed no change in A1c, FPG, or adverse events; meta-analysis of switching trials shows therapeutic equivalence.

Switching between basal products

Glargine ↔ glargine (any brand/biosimilar): 1:1, no dose change.
Tight control or hypoglycemia risk: consider a 10–20% dose reduction on any switch and recheck in ~1 week.
NPH → glargine/detemir/degludec: reduce total basal ~20% (especially if NPH was BID), then titrate.
U-300 glargine (Toujeo) is not 1:1 in practice — it is less potent unit-for-unit than U-100; expect to titrate up, and never assume a straight swap.
Re-teach the device whenever the product changes — pen mechanics and dose windows differ.

Starting insulin in T2DM

Basal first. Start 10 units daily or 0.1–0.2 units/kg/day; use the higher end with marked hyperglycemia or A1c far above goal. See the TDD starter.
Titrate to fasting glucose — increase ~2 units every 3 days (or per a written patient algorithm) until fasting is at target. Give the patient the titration rule in writing; untitrated basal is the most common failure.
Continue metformin and keep a GLP-1 RA / SGLT2i on board where indicated — they lower the insulin dose required and carry cardiorenal benefit.
Over-basalization: basal above ~0.5 units/kg/day, large bedtime-to-morning glucose swings, or hypoglycemia means stop pushing basal and add prandial coverage or a GLP-1 RA instead.
Prandial: start 4 units (or 10% of basal) at the largest meal, titrate, then add remaining meals as needed. See the correction factor / ICR tool.

Writing the Rx so it fills

Sig: "Inject __ units subcutaneously once daily at bedtime" — no "as directed"; pharmacies and plans reject it and days-supply cannot be computed.
Quantity by container, not by units. Pens come 5 per box (3 mL each); vials are 10 mL. A 30-day supply must round to whole containers.
In-use expiry drives quantity at low doses. Most opened pens/vials are discarded 28 days after first use (Tresiba/Toujeo 56 days). A patient on 10 units/day still needs a fresh container monthly, even though the units would last far longer — say so in the Rx note so the plan doesn't reject as "early refill."
Supplies are separate prescriptions: pen needles (e.g. 32G × 4 mm, 1 per injection) or U-100 syringes matched to dose volume; plus test strips and lancets with a frequency that matches the regimen (Medicare ties strip quantity to insulin use).
Refills: send 90 days when the regimen is stable and the plan allows — fewer gaps, fewer stockouts.
Glucagon for anyone on insulin with hypoglycemia risk — nasal or auto-injector, and confirm the caregiver knows where it is.

Safety

Write "units" in full — never "U" or "IU" (10U reads as 100).
U-500 regular is 5× concentrated: dedicated U-500 syringe or pen only, never a U-100 syringe, and independent double-check of the dose.
Never share pens between patients even with a new needle.
Basal insulin does not stop during illness or fasting in T1DM; hold or reduce prandial instead. Give sick-day rules in writing.
CKD / eGFR falling / weight loss / poor intake: insulin clearance drops — reduce dose proactively rather than after a hypoglycemic event.
Older adults and limited life expectancy: loosen the A1c target and de-intensify; a "normal" A1c on insulin is a hypoglycemia signal, not a win.
Confirm the patient can see the dose window, hear the clicks, and physically hold the pen before assuming adherence.

Sources

Marrison ST, Bragg S, Tran E. Type 2 Diabetes: Outpatient Insulin Management. Am Fam Physician. 2026.
ADA Professional Practice Committee. 9. Pharmacologic Approaches to Glycemic Treatment: Standards of Care in Diabetes—2026. Diabetes Care. 2026.
Matli MC, Wilson AB, Rappsilber LM, et al. The First Interchangeable Biosimilar Insulin: Insulin Glargine-Yfgn. J Diabetes Sci Technol. 2023.
Watanabe JH, Strand MW, Shen W, et al. Insulin Glargine Utilization and Spending Before and After the First Biosimilar Insulin Glargine. J Gen Intern Med. 2025.
Kakani P, Katcher BA, Maini L. Improved Insurance Coverage Increased Biosimilar Semglee's Market Share After the FDA's Interchangeability Designation. Health Aff. 2025.
Murphy SJ, Holtkamp NC. Prescription Dispensing for Insulin Glargine After Interchangeable Biosimilar Designation. JAMA Health Forum. 2025.
Xing X, Zhao L, Wang K, et al. Therapeutic equivalence and switching between biosimilar and reference insulins: systematic review and meta-analysis. Diabetes Obes Metab. 2026.
La. Admin. Code tit. 46, pt. LIII, §2517 — Prescription Dispensing; Equivalent Drug Product Interchange. La. R.S. 37:1164(18), 37:1182. 2015 La. Acts (HB 319).