If you’re taking insulin and your blood sugar still won’t come down, something is interfering with how your body absorbs or uses it. This is more common than most people realize, and the cause is rarely just “you need more insulin.” The problem usually traces back to one of several specific, fixable issues ranging from where you inject to how you store your vials to what else is happening in your body that day.
Your Injection Site May Be the Problem
One of the most overlooked reasons insulin stops working well is a condition called lipohypertrophy: rubbery, thickened lumps of fat tissue that form under the skin when you inject in the same spot repeatedly. These lumps feel familiar and often painless, which is exactly why people keep using them. But insulin absorbed through lipohypertrophy enters your bloodstream erratically and inefficiently. Studies show that injecting into these damaged areas requires 27 to 30 percent more insulin to get the same blood sugar reduction you’d get from healthy tissue. If you’ve been gradually increasing your doses and still seeing high numbers, run your fingers over your usual injection sites and feel for any firm or swollen areas.
Where on your body you inject also matters. Insulin absorbs fastest from the abdomen, particularly above the belly button. The arm absorbs it somewhat slower, and the thigh and hip are the slowest. If you switched injection locations recently, or if you tend to favor your thighs, your insulin may simply be arriving later than your blood sugar needs it. For mealtime doses especially, the abdomen gives you the most predictable, fastest absorption.
Your Insulin May Have Lost Its Potency
Insulin is a protein, and proteins break down when exposed to heat, cold, or time. According to the FDA, unopened insulin should be stored in a refrigerator between 36°F and 46°F. Once opened, a vial or pen can stay at room temperature (59°F to 86°F) for up to 28 days and still work. After that window, potency drops even if the insulin looks perfectly clear.
Extreme temperatures accelerate this breakdown. If your insulin sat in a hot car, near a sunny window, or froze in a checked bag on a flight, it may have lost significant effectiveness with no visible change. Insulin in a pump infusion set should be replaced every 48 hours, and discarded immediately if exposed to temperatures above 98.6°F. If your blood sugar has been creeping up and nothing else has changed, try a fresh vial or pen from the refrigerator before adjusting your doses.
Illness and Infection Drive Blood Sugar Up
When your body fights an infection or deals with physical stress, it floods your bloodstream with cortisol, adrenaline, and inflammatory molecules. These stress hormones trigger your liver to dump extra glucose into your blood while simultaneously making your cells less responsive to insulin. It’s a double hit: more sugar coming in and less ability to clear it out. Even a mild cold, a urinary tract infection, or a dental abscess can raise your blood sugar significantly for days.
This isn’t a sign your insulin has stopped working permanently. It means your body’s temporary demand for insulin has outpaced your usual dose. People with diabetes are often advised to have a “sick day plan” with adjusted doses for exactly this reason. If your blood sugar has been stubbornly high and you also feel unwell, the illness itself is likely a major contributor.
Medications That Fight Your Insulin
Steroids are the most common medication culprit. Drugs like prednisone, dexamethasone, and cortisone injections directly increase your liver’s glucose production while making your muscle and fat cells resist insulin. They also promote fat buildup in the liver, compounding the resistance further. Even a short course of oral steroids for an asthma flare or joint inflammation can send blood sugar soaring for days or weeks. Chronic steroid use can push some people from manageable insulin resistance into full diabetes.
Other medications that can raise blood sugar include certain antipsychotics, some blood pressure drugs (particularly thiazide diuretics and beta-blockers), and niacin supplements. If your blood sugar became harder to control around the time you started a new medication, that connection is worth investigating with your prescriber.
The Dawn Phenomenon and Overnight Rebounds
If your blood sugar is specifically high in the morning, two distinct patterns could explain it. The dawn phenomenon happens because your body naturally releases cortisol and growth hormone in the early morning hours to prepare you for waking up. These hormones raise blood sugar, and if your overnight insulin coverage is insufficient, you wake up with numbers that seem impossibly high despite doing everything right the night before.
The Somogyi effect is different. This happens when your blood sugar drops too low overnight, triggering a panic response. Your body releases adrenaline, glucagon, cortisol, and growth hormone all at once, which causes a dramatic rebound spike by morning. The tricky part is that both patterns look identical when you check your morning glucose. The difference is what happened at 2 or 3 a.m. A continuous glucose monitor reveals this clearly. Without one, checking your blood sugar in the middle of the night for a few days can help distinguish between the two, since the treatments are opposite: the dawn phenomenon needs more insulin, while the Somogyi effect needs less.
Timing Gaps Between Insulin and Food
Rapid-acting insulins like lispro (Humalog) and aspart (Novolog) take 10 to 20 minutes to start working and don’t peak for 30 to 90 minutes. Even the newer ultra-rapid formulations like Fiasp and Lyumjev, which start working in 15 to 20 minutes, don’t reach peak effect until about 57 to 63 minutes after injection. Meanwhile, simple carbohydrates from white bread, juice, or rice can hit your bloodstream in under 15 minutes.
If you inject your mealtime insulin right as you start eating, or worse, after, your blood sugar will spike before the insulin catches up. For many people, dosing 15 to 20 minutes before a meal dramatically improves post-meal numbers. The size and composition of the meal matters too. High-fat meals delay digestion but can cause a prolonged, stubborn rise hours later that a single pre-meal dose may not fully cover.
Your Correction Dose May Be Too Small
The amount one unit of insulin lowers your blood sugar is called your insulin sensitivity factor. It’s calculated by dividing 1,800 (for rapid-acting insulin) or 1,500 (for short-acting insulin) by your total daily dose. So if you take 50 units per day of rapid-acting insulin, one correction unit should lower your blood sugar by about 36 mg/dL.
But this number isn’t fixed. It shifts with weight changes, activity levels, stress, hormonal cycles, and the progression of insulin resistance over time. If your correction doses consistently fail to bring your blood sugar down after three to four hours, your sensitivity factor has likely changed and your correction ratio needs updating. Tracking your numbers for a few days and noting how much one unit actually moves your glucose gives you real data to work with.
Underlying Insulin Resistance
In type 2 diabetes, the core issue is that your cells have become less responsive to insulin’s signal. Insulin works by triggering a chain reaction inside your cells that ultimately moves glucose transporters to the cell surface so sugar can enter. When this signaling chain is impaired, those transporters never make it to the surface, and glucose stays trapped in your bloodstream regardless of how much insulin is circulating.
Excess body fat, particularly around the organs (visceral fat), chronic inflammation, and sustained high blood sugar itself all worsen this resistance over time. It creates a frustrating cycle: high blood sugar damages the insulin signaling pathway, which makes blood sugar harder to control, which causes more damage. Severe insulin resistance is sometimes defined as needing 200 or more units per day. At that level, adding more insulin alone yields diminishing returns, and strategies that address the resistance itself, like physical activity, weight loss, or medications that improve insulin sensitivity, become essential.
Dehydration and High Blood Sugar Feed Each Other
When blood sugar is very high, your kidneys try to flush the excess glucose by pulling water from your body. This makes you urinate more, which dehydrates you, which concentrates the remaining sugar in a smaller volume of blood, which pushes your numbers even higher. Insulin has a harder time working in a dehydrated state because reduced blood flow slows its distribution from the injection site. Drinking water won’t lower blood sugar on its own, but staying well hydrated helps your insulin reach where it needs to go and supports your kidneys in clearing excess glucose.