Metformin Side Effects: What You Need to Know Before Taking It

When you take metformin, a first-line medication for type 2 diabetes that lowers blood sugar by reducing liver glucose production and improving insulin sensitivity. It’s one of the most prescribed drugs in the world, used by millions because it works, is cheap, and doesn’t cause weight gain. But like all medications, it’s not without its downsides.

One of the most common issues is metformin diarrhea, a gastrointestinal side effect that affects up to 25% of users, especially when starting the drug or increasing the dose. It’s not just an inconvenience—it can make people stop taking it altogether. The good news? It often fades after a few weeks, and switching to the extended-release version helps most people. Another hidden effect is vitamin B12 deficiency, a long-term risk that’s often overlooked by doctors, even though metformin can lower B12 levels by up to 30% after two years. Low B12 leads to fatigue, nerve damage, and brain fog—all things you might blame on aging or stress, not your diabetes pill.

Some people take metformin for weight loss, even without diabetes. It doesn’t burn fat directly, but it helps reduce appetite and improves how your body handles sugar, which can lead to modest weight loss. That’s why it’s sometimes used off-label, but it’s not a magic solution. If you’re not diabetic and considering it, talk to your doctor first. There’s also a rare but serious risk called lactic acidosis, especially if you have kidney problems, heart failure, or drink alcohol heavily. It’s rare—less than 1 in 10,000—but it’s life-threatening, so knowing the signs (muscle pain, trouble breathing, dizziness) matters.

You won’t find every possible side effect listed in a brochure, but the real-world experiences—like nausea on an empty stomach, metallic taste, or feeling wiped out after meals—are what matter most. The posts below cover what patients actually deal with: how to reduce stomach upset, why some people gain weight instead of losing it, how to test for B12 deficiency, and what to do if metformin stops working. You’ll also see how it stacks up against newer drugs like GLP-1 agonists, and why some people switch. This isn’t just a list of warnings—it’s a practical guide to using metformin safely, effectively, and without surprises.