What insulin stacking is, and why it causes delayed lows
Correcting again before the last dose has finished puts two lots of insulin to work at once. Why the low arrives later.
Insulin stacking is when you take a dose before the last one has finished working, so two lots of fast-acting insulin are active at the same time and push you lower than you meant to go. It usually happens when a number won’t budge and you correct again too soon.
- Fast-acting insulin keeps working for hours after you take it, not just for the first 20 minutes when you feel nothing happening.
- Correcting a stubborn high before the earlier dose has finished is the most common way people stack without noticing.
- The low often lands later, once both doses hit at once, which is why it feels like it came out of nowhere.
- How long a dose keeps working depends on your insulin type: your team will have told you what to expect from yours.
- This is common, it’s not a mistake you got wrong, and knowing the timing is what stops it repeating.
Why a dose you took an hour ago is still working
When you take fast-acting insulin (the kind you take with food or to bring a high down), it doesn’t act all at once and then stop. It builds up, peaks, then tails off over a few hours. The exact shape varies by insulin, but the tail is longer than most people expect.
The problem is that the strongest effect often comes well after you’ve stopped feeling like anything is happening. So you take a dose, your glucose is still high 45 minutes later, it feels like nothing worked, and you take more. In reality the first dose hadn’t finished. Now two doses are working together, and when they overlap you drop further and faster than either one on its own.
What this changes about chasing a high
The practical shift is patience with corrections. A high number after a meal is often the first dose still working its way through, not a sign it failed. Piling another dose on top is what turns a manageable high into a hypo (a low, glucose below about 4 mmol/L) a couple of hours later.
The useful thing to know is how much insulin you’ve already got active, sometimes called insulin on board. Many pumps track this for you and show it on screen. On injections you don’t have that display, so writing down when you dosed and how much gives you the same information: if the last dose was recent, the high is probably still coming down on its own.
What changes how long a dose lasts
Two people on the same insulin can see it act over different lengths of time, and the same person can vary day to day. A bigger dose tends to hang around longer. Movement matters in both directions: steady activity speeds insulin up and can deepen a low, but intense or competitive effort (a hard sprint session, a match, a heavy lift) can push glucose up for a while through the stress response, which makes a high look like it needs correcting when it doesn’t. Injection site and warmth change absorption too. None of this is a flaw in how you’re doing it, it’s just why there’s no single number for how long to wait, and why your team’s guidance on your particular insulin matters more than a general rule.
How stacking tends to show up
Examples, not instructions or doses.
You dosed for dinner, and an hour later you’re 11 mmol/L and it’s not moving. It’s tempting to correct straight away. Often that reading is the meal dose still working, and adding another lands you low by bedtime.
You corrected a high mid-afternoon, checked again 40 minutes later, saw little change and corrected again. Both doses peak together later, and you’re treating a low at teatime you didn’t see coming.
You feel fine, then drop quickly with no obvious trigger. Looking back, there were two doses close together earlier, and this is where they overlapped.