Grapefruit and grapefruit juice block an enzyme in the gut that breaks down certain medicines, which can cause higher-than-intended drug levels. With statins such as simvastatin and, to a lesser degree, atorvastatin, this can increase the risk of muscle side effects, including a rare but serious condition affecting the muscles and kidneys.
Not all statins are affected: pravastatin, rosuvastatin, and fluvastatin are largely unaffected by grapefruit, so they may be alternatives for people who enjoy it. The amount of grapefruit that matters varies, so check the leaflet for your specific statin.
Ask your pharmacist whether your statin interacts with grapefruit. Seek medical advice promptly if you develop unexplained muscle pain, tenderness, weakness, or dark-coloured urine.