Lantus and Toujeo are long-acting forms of insulin glargine designed to provide background (basal) insulin throughout the day and night. Although they are officially approved for once-daily use, real-world physiology tells a more nuanced story. Their half-lives, absorption patterns and day-to-day variability mean that many people actually achieve smoother glucose control with twice-daily dosing. In this article, we explain the evidence, the practical implications, and how to choose the timing that best fits your glucose patterns, lifestyle and safety.
What are Lantus and Toujeo?
Both Lantus (insulin glargine U100) and Toujeo (insulin glargine U300) contain the same active ingredient. The difference is concentration and how the insulin is released from the injection site:
- Lantus (U100): typically lasts up to 22–24 hours in regulatory studies.
- Toujeo (U300): flatter and longer profile, lasting up to 30–36 hours in studies, though real-world duration varies considerably.
In theory they provide a slow, steady insulin release to counter your liver’s glucose output overnight and between meals. In practice, their true duration depends heavily on dose, body composition, injection site, and individual absorption patterns.
For background, you may find these helpful: Lantus SoloStar (insulin glargine) and How is diabetes treated?
The real pharmacology: half-life and variability
A crucial concept is that half-life is not the same as duration of action. Half-life is how long it takes for the insulin concentration to fall by 50%. For Lantus and Toujeo:
- Lantus half-life ≈ 12 hours
- Toujeo half-life ≈ 14–18 hours (with wide variability)
These numbers indicate that for many adults the insulin level begins to drop meaningfully before 24 hours has elapsed. That is why patients who struggle with fasting glucose or late-day drift often do better on split dosing.
Furthermore, absorption varies:
- Across days in the same person
- Between individuals
- With larger single doses (over ~50 units)
This variability is one of the key reasons why a rigid once-daily schedule doesn’t always provide stable coverage.
Why once-daily dosing is often not enough
Although both insulins are approved for once-daily use, clinical experience shows that many adults do not receive full 24-hour coverage. Signs that once-daily dosing may not be adequate include:
- Rising glucose late afternoon or early evening
- High fasting glucose despite seemingly large basal doses
- Glucose variability that improves dramatically once split
- Total daily basal dose above 50 units
For these individuals, splitting the dose into morning and evening can improve absorption consistency, smooth glucose levels and reduce basal “peaking”.
Once-daily approval: regulatory vs physiological reality
When Lantus and later Toujeo were developed, the goal of the registration studies was to show that they were at least as effective as older insulins given twice daily. Demonstrating once-daily dosing was strategically advantageous—simpler for patients and more competitive for the manufacturers.
However, regulatory approval does not change human physiology. The evidence generated to achieve a one-dose-per-day label does not mean that all people will achieve reliable 24-hour coverage from a single injection. Many do—but many do not—and this is important for people with diabetes to understand.
Morning vs evening dosing: the real pros and cons
The product information allows Lantus/Toujeo to be taken once daily at any time. In practice, morning and evening doses behave very differently.
Morning dosing
Pros
- Easy to attach to a morning routine
- Convenient for clinic review of technique
- Feels reassuring for people worried about overnight hypoglycaemia
Cons — particularly important:
- By the following morning, much of the insulin has worn off
- Fasting glucose rises → titration algorithm increases the dose
- The larger dose peaks earlier in the day
- This creates a higher risk of afternoon/evening hypoglycaemia
Evening or bedtime dosing
Pros
- Peak activity aligns with liver glucose output overnight
- Supports stable fasting glucose
- Titration adjusts the dose at a time when the insulin is actually active
- Often smoother daytime patterns
Cons
- Less suitable for people with extremely irregular evening routines
- People living alone with severe hypoglycaemia risk may require tailored plans
Why bedtime dosing is often the best compromise
For most adults with type 1 or type 2 diabetes, the greatest glucose challenge is overnight and early morning. A bedtime basal insulin dose:
- Matches the body’s nocturnal glucose output
- Targets the “dawn phenomenon”
- Prevents high fasting readings
- Reduces the risk of daytime “peak” lows
This is why many endocrinologists prefer a bedtime dose as the starting point, then adjust based on CGM or glucose patterns.
When and why to split Lantus or Toujeo
Many people achieve better control and fewer side effects when taking Lantus or Toujeo twice daily. Splitting may help when:
- You require more than 50 units/day
- Your glucose rises late afternoon/evening
- Your fasting levels are high despite increasing once-daily doses
- You experience lows during the day after increasing a morning dose
- Your absorption is variable and large single injections are less predictable
Splitting the daily dose (e.g. 50/50 or 60/40) can:
- Smooth out peaks and troughs
- Provide more reliable 24-hour coverage
- Improve comfort with large doses
- Reduce hypoglycaemia from daytime “peaking”
How Tresiba (insulin degludec) differs
Tresiba is an ultra-long-acting basal insulin with a half-life of approximately 25 hours and a duration of action beyond 42 hours. This makes it much more forgiving of:
- Irregular dosing times
- Missed or delayed doses
- Day-to-day absorption variability
Tresiba behaves very differently from glargine, and is generally a true once-daily insulin unless clinically indicated otherwise.
How to safely change your basal insulin timing
Because glargine insulins have long activity tails, switching timing abruptly may cause overlap or gaps. Clinicians typically use one of these approaches:
- Move your dose by 1–2 hours per day until you reach the desired time
- Occasionally use a one-off half dose when moving from morning to evening (or vice versa)
- Increase glucose monitoring temporarily
Does timing differ for type 1 and type 2 diabetes?
Type 1 diabetes
Timing should support:
- Stable overnight glucose
- Minimal daytime variability
- Predictable basal–bolus flexibility
Bedtime dosing is common, but CGM-guided individualisation is key.
Type 2 diabetes
Basal insulin is usually titrated to fasting glucose. Because fasting control is the main target, bedtime dosing generally makes the most physiological sense.
Common mistakes to avoid
- Taking the dose at unpredictable times
- Doubling up after a missed dose
- Increasing basal insulin to “fix” every high glucose
- Stopping basal when starting a GLP-1 or SGLT2 inhibitor
- Not checking fasting glucose when titrating doses
When to talk to your diabetes team
- Repeated hypoglycaemia, especially late afternoon or overnight
- Fasting glucose that remains high despite consistent use
- Questions about switching to or from Toujeo, Lantus or Tresiba
- Life changes: shift work, steroids, pregnancy, Ramadan, travel
Useful links and further reading
- American Diabetes Association (ADA) – Insulin Basics
- NIDDK – Taking Insulin
- International Diabetes Federation – idf.org
- NHS (UK) – Insulin medicines (patient information)
- Regulatory product information:
