The amount of inhibiting substance in grapefruit juice may vary by brand, concentrations and storage conditions. Regular grapefruit juice is prepared by diluting frozen grapefruit juice with normal amount of water. Double-strength grapefruit juice is prepared by diluting frozen grapefruit juice with half the normal amount of water. One glass of regular grapefruit juice is enough to significantly raise the blood levels of the number of drugs. The inhibitory effect of grapefruit juice on CYP 3A4 can last for several hours.
Grapefruit juice increases the blood levels of extended release preparations.
CYP 3A4 is involved in the metabolism of many medications. When CYP 3A4 is inhibited by grapefruit juice, the blood levels of many medications may rise. Examples of drugs reported to interact are listed in the table below. Since many more drugs are likely to interact, it is important to ask your doctor or pharmacist if you drink grapefruit juice and are taking any medications. Flushing headache, increased heart rate decreased blood pressure Decreased heart rate, constipation, decreased blood pressure 2.