Usually you can have a small amount of alcohol with metformin, but heavy or binge drinking is risky — keep within strict limits and ask your doctor how much is safe for you.
Metformin is not absolutely off-limits with alcohol, but drinking raises two real risks. MedlinePlus (FDA-derived labeling) states that "drinking alcohol increases your risk of developing lactic acidosis or may cause a decrease in blood sugar" — lactic acidosis is a rare but potentially life-threatening buildup of lactic acid, and alcohol can also amplify metformin's blood-sugar-lowering effect. The danger rises sharply with binge drinking and in people who already drink heavily, which is why the labeling and NHS both single out heavy/regular drinkers. The NHS advises no more than 2 units in one day and staying within the UK guideline of 14 units a week, because exceeding this increases the risk of low blood sugar.
Keep to the NHS limits: no more than 2 units on any day and no more than 14 units a week, and avoid binge drinking (large amounts in a short time). Tell your doctor before starting metformin if you drink a lot or sometimes drink heavily — this can make metformin unsuitable. Higher-risk people who should be especially cautious or avoid alcohol include those with kidney disease, liver disease, or heart disease. Watch for warning signs of lactic acidosis and stop the medicine and seek emergency care if they appear: extreme tiredness or weakness, nausea and vomiting, stomach pain, deep or rapid breathing or shortness of breath, dizziness, an unusually fast or slow heartbeat, muscle pain, or feeling cold (especially in the hands or feet). Also watch for low blood sugar (shakiness, sweating, confusion). Ask your doctor or pharmacist how much alcohol, if any, is safe for your individual situation.
This is general reference, not medical advice, and not a guarantee of safety. Interactions depend on your doses, health conditions, and other medicines. Always confirm with your pharmacist or doctor before combining products, and follow the dosing on each label.