High homocysteine means your blood contains too much of an amino acid that, at elevated levels, can damage blood vessels and raise your risk for heart disease, stroke, and other serious conditions. A normal level falls between 5 and 15 micromoles per liter (mcmol/L). Levels between 15 and 30 are considered mildly elevated, 30 to 100 is moderate, and anything above 100 is severe.
Why Homocysteine Levels Rise
Your body produces homocysteine as a byproduct of breaking down protein. Normally, B vitamins recycle it into other useful compounds before it accumulates. Folate (vitamin B9) plays the biggest role in keeping levels low, with vitamins B12 and B6 acting as essential helpers in the same recycling process. When any of these vitamins are deficient, homocysteine builds up in the bloodstream instead of being cleared.
Genetics also plays a significant part. A common gene variant called MTHFR affects how well your body uses folate. Two specific versions of this variant, known as C677T and A1298C, can slow folate processing enough to raise homocysteine. Having two copies of C677T, or one copy of each variant, is the combination most likely to cause a noticeable increase. About 10 to 15 percent of people carry gene combinations that meaningfully affect their levels.
Lifestyle factors push levels higher as well. Coffee consumption has a clear dose-response relationship with homocysteine, likely because caffeine interferes with how your body uses B6. Smoking raises levels, particularly in women, partly through lowering folate intake. Heavy alcohol use also increases homocysteine. Kidney disease, hypothyroidism, and certain medications can contribute too.
How It Damages Your Body
Elevated homocysteine injures the inner lining of blood vessels through several overlapping mechanisms. It triggers inflammation, promotes the accumulation of damaging molecules called free radicals, and interferes with nitric oxide, a compound your arteries rely on to stay relaxed and flexible. Over time, this damage makes arteries stiffer and more prone to plaque buildup, the process behind heart attacks and strokes.
The effects reach beyond the cardiovascular system. A landmark study published in the New England Journal of Medicine found that each 5 mcmol/L increase in homocysteine raised the risk of Alzheimer’s disease by 40 percent. People with levels above 14 mcmol/L had roughly double the risk of developing dementia compared to those with lower levels.
Bone health is affected too. Research following older adults found that those with homocysteine in the highest quartile for their age had 1.9 times the fracture risk of those with lower levels. This association held even after accounting for bone mineral density, meaning homocysteine appears to weaken bones through a mechanism beyond simple bone thinning.
Symptoms You Might Notice
High homocysteine itself typically causes no obvious symptoms. Most people discover it through a blood test ordered for another reason. This is what makes it tricky: the damage accumulates silently over years.
When symptoms do appear, they usually reflect the underlying cause rather than the homocysteine itself. If B12 or folate deficiency is driving the elevation, you might experience fatigue, numbness or tingling in your hands and feet, weight loss, or brain fog. In rare cases, a genetic condition called homocystinuria causes severely high levels from birth, leading to vision problems (particularly displaced lenses), developmental delays in children, and early cardiovascular disease in young adults.
Who Gets Tested
Homocysteine testing is not part of routine screening. Both the American Heart Association and the European Society of Cardiology recommend against checking it in the general population. Instead, doctors typically order the test in specific situations: unexplained blood clots in younger people, early heart disease without traditional risk factors, suspected B12 or folate deficiency, or a family history of very high homocysteine.
If your level comes back elevated, your doctor will often check your folate, B12, and B6 status as a next step, and may order MTHFR gene testing if a genetic cause is suspected.
Lowering Your Levels
The most effective way to bring homocysteine down is increasing your intake of folate, B12, and B6. For many people, dietary changes alone make a meaningful difference. Dark leafy greens, legumes, fortified grains, eggs, meat, and fish are rich in these vitamins. A diet low in processed food and high in vegetables covers most of the bases.
When diet isn’t enough, supplementation with folic acid and B12 reliably lowers levels. People with MTHFR variants sometimes benefit from methylfolate, a pre-converted form of folate that bypasses the genetic bottleneck. Your doctor can help determine the right approach based on your specific situation.
There’s an important nuance here, though. While B-vitamin supplementation consistently lowers homocysteine numbers on lab tests, large clinical trials have not shown that lowering homocysteine through supplements directly reduces heart attack or stroke risk. This is why the AHA does not recommend B-vitamin supplementation specifically for cardiovascular prevention. The current thinking treats homocysteine as one piece of a larger metabolic picture rather than a standalone target to treat.
Lifestyle changes help too. Cutting back on coffee, quitting smoking, and moderating alcohol intake all contribute to lower levels. Regular physical activity and maintaining a healthy weight support the same metabolic pathways that keep homocysteine in check.