Metformin Long-Term Use: What Happens After Years on the Drug

When you take metformin, a first-line oral medication for type 2 diabetes that lowers blood sugar by reducing liver glucose production and improving insulin sensitivity. It's been the go-to drug for decades because it works, doesn't cause weight gain, and is cheap. Also known as Glucophage, it's prescribed to millions worldwide — but most people never ask what happens after five, ten, or twenty years of daily use.

Long-term metformin use isn't harmless. It quietly affects your body in ways many doctors don't talk about. One big issue is vitamin B12 deficiency, a common but underdiagnosed side effect of prolonged metformin use that can cause nerve damage, fatigue, and brain fog. A 2010 study in Diabetes Care found that 13% of people on metformin for over four years had low B12 levels — and the longer they took it, the worse it got. This isn't rare. It's expected. Yet, most patients aren't tested unless they're already showing symptoms. Another hidden risk is how it interacts with your kidney function, the organ that clears metformin from your body. As you age, kidneys slow down. Metformin builds up. That’s why doctors check your eGFR regularly — but many don’t adjust doses early enough, putting you at risk for lactic acidosis, a rare but deadly condition.

On the flip side, long-term metformin use might be doing more than just controlling blood sugar. Studies suggest it could lower your risk of certain cancers, like colon and breast cancer, possibly by reducing insulin spikes that feed tumor growth. Some research even links it to slower aging and longer lifespan — but these are still being studied. What we know for sure is this: metformin isn't a magic bullet, and it's not meant to be taken forever without oversight.

People who’ve been on metformin for years often feel fine — until they don’t. Numb toes, constant tiredness, memory lapses — these aren’t just "getting older." They could be signs your body’s running low on B12 or struggling to process the drug. Your doctor should be checking your B12 levels at least once a year, monitoring your kidney function, and asking if you’re still taking it as prescribed. If you’re not getting these checks, you’re not getting full care.

Below, you’ll find real stories and science-backed facts about what happens when metformin becomes part of your daily life for years. From drug interactions to hidden deficiencies, these posts cover what most guides leave out — because your health shouldn’t depend on guesswork.