This guide was written by our editorial team from public health sources, not by a clinician, and it is general education, not medical advice. Primary Care Evidence Guide is an independent health information publication. It is not a medical practice and does not provide healthcare services, and this web address was previously used by a healthcare provider with no connection to this publication.
The Short Answer
High blood sugar (hyperglycemia) has more than one cause. The main long-term driver is diabetes itself — insulin resistance in type 2 diabetes, or the loss of insulin production in type 1. But a single high reading can also come from illness, certain medicines, physical stress, or test conditions — and none of those, on their own, mean you have diabetes. Eating sugar is not the single cause of diabetes; it's one input into a larger system involving your pancreas, how your cells respond to insulin, your weight, your genetics, and sometimes your immune system.
Below: which illnesses and medicines are documented to raise glucose, why “too much sugar” doesn't fully explain diabetes, how diabetes types differ from a single out-of-range number, and a short log to help a clinician sort out what's going on.
Illnesses and Medicines That Can Raise Blood Sugar
Illness and infection
Being sick is a documented trigger for higher glucose, including in people who already manage diabetes carefully. The CDC names illness as one of the two most common causes of diabetic ketoacidosis (DKA), a serious high-sugar emergency — noting that illness can make it harder to eat or drink normally, which in turn makes blood sugar harder to manage. The same CDC page lists other documented DKA triggers: missed insulin doses or a malfunctioning insulin pump, heart attack or stroke, physical injury, and alcohol or drug use. These are specifically documented triggers in people who already have diabetes — this is not a claim that illness alone causes diabetes in someone who doesn't have it.
Certain medicines
That same CDC source names corticosteroids (used to treat inflammation) and some diuretics among the medicines linked to DKA risk. Separately, a peer-reviewed narrative review indexed on PubMed Central describes corticosteroids as the leading cause of drug-induced hyperglycemia generally (Liu et al., World Journal of Diabetes, 2015). If you're on a steroid course — oral, injected, or a long inhaled regimen — and notice higher readings, that's a known, documented side effect worth raising with the prescriber, not something to try to manage through diet alone.
Physical stress
Severe physical stress — illness, trauma, surgery — is also associated with a temporary rise in glucose even in people without a diabetes diagnosis, sometimes called stress hyperglycemia; clinical literature describes this pattern as typically resolving once the underlying stress resolves. If you notice elevated readings only during an acute illness or after a major physical event, that pattern itself is useful information to bring to a clinician rather than something to interpret alone.
Does Eating Sugar Alone Cause Diabetes?
No — and this is one of the most common misunderstandings people bring to a visit. Diabetes is a disease of how your body regulates glucose, not simply a tally of sugar consumed. NIDDK explains that in type 1 diabetes, the immune system attacks the insulin-producing cells in the pancreas — an autoimmune process unrelated to diet. In type 2 diabetes, the far more common form, cells stop responding normally to insulin, and the pancreas eventually can't keep up with the extra insulin it produces to compensate.
Diet is one contributor to that picture, but CDC's listed risk factors for type 2 diabetes go well beyond sugar intake: having prediabetes, carrying excess weight, being 45 or older, having a parent or sibling with type 2 diabetes, being physically active fewer than three times a week, a history of gestational diabetes or giving birth to a baby over nine pounds, non-alcoholic fatty liver disease, and belonging to certain racial or ethnic groups CDC names as higher-risk. For more on why insulin resistance specifically can persist despite dietary changes, see our related piece on the metabolic factors behind stubborn blood sugar.
Diabetes Types, Prediabetes, and a Single Reading Are Not the Same Thing
It's worth separating three things people often collapse into one:
- Diabetes types. NIDDK describes type 1 (autoimmune, usually diagnosed in children and young adults but possible at any age), type 2 (the most common form, linked to insulin resistance), and gestational diabetes (develops during pregnancy and usually resolves after birth, though it raises future type 2 risk). Less common forms include monogenic diabetes, caused by a single gene change, and diabetes resulting from pancreas damage or pancreas surgery.
- Prediabetes. Blood glucose higher than normal but not yet in the diabetes range — a risk marker, not a diagnosis of diabetes itself.
- A single reading. One number from a home meter reflects that moment — test technique, recent food, illness, medicines, and stress can all move it. It's a data point, not a diagnosis. We cover how to document and interpret an unexpected number in full in this companion guide, including a complete context log and emergency criteria.
Confirming an actual diagnosis takes lab testing — a fasting plasma glucose test, an A1C test (reflecting roughly three months of average glucose), or a random plasma glucose test when symptoms are present — interpreted by a clinician who knows your history, not a home meter reading on its own.
A Short Causes-and-Context Log
If you've noticed a pattern of higher readings and want to bring something useful to an appointment, this shorter log focuses specifically on causes — not full testing technique (see the companion log linked above for that). Copy these fields into a notes app or notebook:
- Test timing: date, time, and how long since you last ate or drank anything besides water
- Food context: what you ate or drank in the hours before, including anything unusual (illness-related appetite changes, alcohol, skipped meals)
- Medicines: anything taken that day, including steroids, diuretics, or any new prescription, with name, dose, and time
- Symptoms: what you felt at the time, including signs of illness, or “none”
- The exact question to bring to your clinician: “Could [this illness / this medicine] explain my recent readings, or should I be tested for diabetes or prediabetes?”
This log does not diagnose anything from a single number. Its job is to give a clinician the context needed to tell a temporary, explainable rise from a pattern worth testing for.
When to Get Care
Some situations call for immediate medical attention rather than logging and waiting. CDC's guidance for people managing diabetes is to seek emergency care for blood sugar at or above 300 mg/dL, fruity-smelling breath, vomiting that won't stop, trouble breathing, or multiple signs of DKA — the full symptom list and emergency criteria are in our out-of-range reading guide. If you don't have a diabetes diagnosis and are experiencing ongoing thirst, frequent urination, blurred vision, or unexplained fatigue, those are reasons to ask your doctor about testing, not symptoms to manage on your own.
Next Step
If a high reading, a new medicine, or a recent illness has you asking why, the goal isn't to self-diagnose — it's to arrive at your next appointment with the right context and the right question. Bring the log above, note anything unusual about timing or medicines, and ask directly whether what you're seeing points to a temporary cause or warrants testing.