Whether grapefruit is a problem depends on which statin you take — it is a real concern with simvastatin (and other statins broken down by the same liver/gut enzyme), a matter of quantity with atorvastatin, and generally not flagged for statins cleared by other pathways, so confirm with your doctor or pharmacist about your specific statin.
Grapefruit and grapefruit juice contain compounds that block an intestinal enzyme (CYP3A4) that normally breaks certain statins down, so more of the drug can enter the blood and raise the risk of the side effect doctors watch for most: muscle damage (myopathy), which in severe cases (rhabdomyolysis) can injure the kidneys — the FDA states that with statins affected this way, drinking a lot of grapefruit juice can leave "too much of the drug" in the body and increase the risk of "liver and muscle damage that can lead to kidney failure" (fda.gov). Critically, the FDA also says "grapefruit juice does not affect all the drugs" in a given category, so the warning is statin-specific rather than blanket. The FDA names simvastatin (Zocor) and atorvastatin (Lipitor) as statins grapefruit juice can interact with, and both drugs' FDA prescribing labels list grapefruit juice among the things that increase the risk of myopathy and rhabdomyolysis (DailyMed). For atorvastatin the label sets a quantity threshold — "concomitant intake of large quantities, more than 1.2 liters daily, of grapefruit juice is not recommended" — and the NHS uses a similar "more than about a litre a day" figure, indicating small or occasional amounts are handled differently than large daily quantities. Because the risk hinges on which statin you take and how much grapefruit you consume, MedlinePlus's consumer guidance for both atorvastatin and simvastatin is simply to "talk to your doctor" about grapefruit before assuming yours is unaffected.
Warning signs to report promptly: unexplained muscle pain, tenderness, or weakness; dark or cola-colored urine; unusual tiredness or lack of energy; or fever/flu-like symptoms — the FDA labels instruct patients to promptly report these as possible signs of muscle injury, and MedlinePlus lists them among serious side effects needing urgent medical attention. Who should be most careful: anyone on simvastatin or a statin broken down by the same enzyme, anyone on a higher statin dose, adults 65 and older, and people with kidney or thyroid problems — the labels list all of these as risk factors for myopathy. On amounts and timing: the sources give quantities, not a clock — for atorvastatin the FDA label flags "more than 1.2 liters daily" and the NHS says "more than about a litre a day," so large daily quantities are the specific concern; the sources do not describe a safe number of hours to wait between grapefruit and your dose, so do not assume that simply spacing them out makes it safe. The FDA also notes that Seville oranges (often used in marmalade), pomelos, and tangelos "might have the same effect as grapefruit juice," so check juice-blend and marmalade labels too. When to call a clinician: before assuming your statin is one of the unaffected ones, or if you want to switch to a statin that grapefruit does not affect, confirm with your doctor or pharmacist — and seek urgent care if you develop severe muscle pain with dark urine.
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.