Several common supplements can raise blood sugar or interfere with how your body processes it, though the list is shorter than you might expect. Niacin and caffeine have the strongest evidence for directly affecting blood sugar levels. A few others don’t actually raise glucose but can make your readings look wrong, which matters just as much if you’re monitoring your levels.
Niacin (Vitamin B3)
Niacin is the supplement most clearly linked to higher blood sugar. It’s widely used to improve cholesterol levels, but it comes with a trade-off: it reduces your body’s ability to respond to insulin. In a study of 12 healthy, non-diabetic people, just two weeks of niacin at doses up to 2 grams per day caused an 18% drop in insulin sensitivity. That reduction correlated with a rebound spike in fatty acids in the blood, which appears to impair how the body metabolizes glucose.
The exact mechanism isn’t fully pinned down, but animal studies show a pattern: niacin initially lowers free fatty acids, then triggers a rebound where fatty acid levels surge back. That rebound promotes insulin resistance, meaning your cells don’t take up glucose as efficiently and blood sugar climbs. Notably, a combined analysis of four clinical trials found that the glucose increase wasn’t limited to high doses or specific formulations of niacin. Whether people took immediate-release or extended-release versions, fasting glucose rose significantly across the board.
If you take niacin for cholesterol and also monitor your blood sugar, this is worth discussing with whoever manages your care. The blood sugar effect is well-documented enough that it’s considered a known side effect of therapeutic-dose niacin.
Caffeine Supplements
Caffeine in pill or powder form can meaningfully reduce insulin sensitivity in the short term. A systematic review and meta-analysis of studies in healthy people found that a single dose of caffeine significantly decreased the insulin sensitivity index compared to placebo. The effect is acute, meaning it happens in the hours after you take it, not as a long-term shift.
This matters most if you’re taking concentrated caffeine supplements rather than drinking coffee. Coffee contains other compounds that may partially offset caffeine’s glucose effects over time, which is why regular coffee drinkers don’t always see the same blood sugar spikes. But isolated caffeine, especially at higher doses, consistently impairs how well your body handles a sugar load. If you take caffeine pills and notice post-meal glucose running higher than expected, the caffeine itself could be a factor.
Supplements That Don’t Actually Raise Blood Sugar
A few supplements have a reputation for affecting glucose, but the clinical evidence doesn’t support the concern.
Glucosamine: This joint supplement has long carried warnings about blood sugar, partly because it’s an amino sugar and the logic seemed plausible. But a double-blind, placebo-controlled trial in non-diabetic osteoarthritis patients found no significant changes after 90 days of standard-dose glucosamine. Fasting blood sugar, glucose tolerance, and insulin resistance all stayed essentially flat. The conclusion was straightforward: oral glucosamine at routine dosages had no meaningful effect on glucose metabolism.
Fish oil: A meta-analysis of 12 randomized controlled trials in people with type 2 diabetes found no significant difference in fasting glucose, fasting insulin, hemoglobin A1c, or insulin resistance between fish oil and placebo groups. Fish oil did improve blood lipid profiles, but it neither raised nor lowered blood sugar in any clinically meaningful way.
DHEA: Despite some theoretical reasons it might affect insulin and glucose, a meta-analysis in elderly men and women found that DHEA supplementation had no effect on blood glucose levels.
Melatonin: Rather than raising blood sugar, melatonin taken before an evening meal actually improved glucose tolerance and insulin sensitivity in a study of healthy young men. Postprandial glucose and insulin levels were both significantly lower with melatonin compared to eating the same late meal without it.
Supplements That Fake High Readings
Some supplements don’t actually raise your blood sugar but interfere with the tools used to measure it. If you rely on a glucose meter or continuous glucose monitor, this distinction is critical because you could make treatment decisions based on numbers that aren’t real.
Vitamin C
High-dose vitamin C is a well-known interferent with continuous glucose monitors. For the Freestyle Libre 2 and Libre 3 systems, doses above 500 mg per day can throw off readings. The Libre 2 Plus has a slightly higher threshold at 1,000 mg per day. The interference comes from the electrochemical sensor technology these devices use. Vitamin C is structurally similar enough to glucose that the sensor can’t always tell them apart, producing falsely elevated numbers. If you take vitamin C supplements and use a CGM, check the manufacturer’s documentation for your specific device.
Biotin
Biotin doesn’t interfere with glucose meters the way vitamin C does, but it can distort laboratory blood tests, including some assays used to measure blood sugar and A1c. Many lab tests rely on a biotin-based chemical reaction as part of their procedure. When you’re supplementing with biotin at doses above 1 mg per day, enough of it circulates in your blood to disrupt that reaction and produce either falsely high or falsely low results depending on the specific test platform.
Normal dietary intake of biotin isn’t enough to cause problems. The issue arises with supplements, which commonly contain 5 to 10 mg per serving, well above the interference threshold. The standard recommendation is to stop taking biotin for at least 48 hours before any blood draw. Hair, skin, and nail supplements are a common source of high-dose biotin that people forget to mention before lab work.
What This Means in Practice
If you’re tracking blood sugar and notice unexplained increases, niacin and caffeine supplements are the two most likely culprits among common over-the-counter products. The effect from niacin is sustained as long as you’re taking it. The effect from caffeine is temporary but can be significant in the hours after a dose.
For everything else, the more common problem is measurement interference rather than a genuine rise in glucose. Knowing which supplements affect your specific monitoring device, and which ones can skew lab results, prevents you from reacting to numbers that don’t reflect what’s actually happening in your body.