Missing a dose of long-acting insulin causes your blood sugar to rise, sometimes significantly, because your body loses the steady background insulin that keeps glucose in check between meals and overnight. How quickly this becomes a problem and what you should do about it depends on whether you have type 1 or type 2 diabetes, how late you are, and which insulin you take.
The 2-Hour Rule for Late Doses
If you realize you missed your dose within about 2 hours of when it was due, you can generally take your full usual dose right away. This is the simplest scenario, and for most people, blood sugar won’t have climbed much in that window.
Beyond 2 hours, things get more nuanced. One common approach for once-daily basal insulin is to take half your normal dose if you’re 3 to 4 or more hours late, then resume your full dose at your next regularly scheduled time. This reduces the risk of overlapping too much insulin in your system while still providing some coverage. If you take basal insulin twice a day and miss one of those doses entirely, you may be better off skipping that dose, monitoring your blood sugar closely, and using small correction doses of rapid-acting insulin (if you have it) until your next scheduled injection.
Your Insulin Brand Matters
Not all long-acting insulins behave the same way after a missed dose. Standard once-daily options like glargine (Lantus, Abasaglar) should be taken as soon as you notice the missed dose. You’ll then need to shift the timing of your next dose and gradually work your way back to your usual schedule over the following days.
Ultra-long-acting insulin (degludec, sold as Tresiba) has a much wider built-in flexibility window of roughly 8 hours. If you’re within that 8-hour window, just take it and carry on. If you’re past the 8-hour mark, take your usual dose when you remember and adjust back toward your normal time. Tresiba’s longer duration in the body means a late dose creates less of a gap in coverage than it would with shorter-duration basal insulins.
What Happens to Your Blood Sugar
Long-acting insulin provides a constant, low level of insulin that suppresses glucose production by your liver and helps your cells absorb sugar throughout the day. Without it, blood sugar starts climbing. For people with type 2 diabetes who still produce some insulin on their own, this rise may be gradual and moderate. You might notice increased thirst, more frequent urination, fatigue, or blurry vision as levels creep up.
For people with type 1 diabetes, the consequences are more immediate and serious. Because the body produces no insulin at all, blood sugar can spike quickly, and the body begins breaking down fat for energy. This process produces ketones, acidic byproducts that build up in the blood. The result can be diabetic ketoacidosis (DKA), a dangerous condition that develops over hours to days. Poor adherence to insulin is the single most common trigger for DKA in young people with type 1 diabetes, accounting for more than half of first DKA episodes and nearly 80% of repeat episodes.
Signs That You Need Emergency Help
After a missed dose, check your blood sugar more often than usual. If you use a continuous glucose monitor, watch it closely for sharp upward trends. If you don’t, test with a finger stick every 2 to 3 hours until your levels stabilize after your next dose.
The CDC recommends going to the emergency room or calling 911 if your blood sugar stays at 300 mg/dL or above, or if you have high ketone levels (which you can check with urine strips or a blood ketone meter). Symptoms of DKA include nausea or vomiting, stomach pain, fruity-smelling breath, rapid breathing, and confusion. These symptoms can escalate quickly, especially in type 1 diabetes.
Never Double Your Next Dose
It might seem logical to take twice as much insulin the next day to “make up” for the missed dose. This is dangerous. An insulin overdose causes severe low blood sugar (hypoglycemia), which can lead to seizures, loss of consciousness, and electrolyte imbalances. With long-acting formulations, the effects of an overdose are especially hard to manage because they persist for 48 to 96 hours. Taking a double dose creates a prolonged period where your blood sugar could crash repeatedly, even with eating.
Getting Back on Schedule
If your late dose shifted your timing by a few hours, you don’t need to snap back to your original schedule immediately. Instead, adjust gradually. For example, if you normally inject at 10 p.m. but took a late dose at 2 a.m., you might take your next dose at midnight, then 11 p.m. the following night, easing back to your usual time over two or three days. This prevents doses from stacking too closely together.
If you find yourself missing doses regularly, a few practical changes can help. Setting a daily phone alarm is the simplest fix. Keeping your insulin and supplies in a visible, consistent spot reduces the chance of forgetting. Some newer insulin pens have memory features that record your last injection time, which removes the guesswork about whether you actually took your dose. Pairing your injection with a daily habit you never skip, like brushing your teeth before bed, can also make it more automatic over time.