On a hot summer day, a cold glass of grapefruit juice or having grapefruit with your breakfast can be refreshing. If you are taking certain medications, however, it can be dangerous. There is a chemical in this fruit and its juice called furanocoumarin, which attaches to an enzyme in your intestines and reduces the absorption of certain medications, says the Harvard School of Medicine Family Health Guide. If you are taking medication, ask your health care provider whether the drug interacts with grapefruit and grapefruit juice.

Calcium Channel Blockers

Calcium channel blockers can be used to treat high blood pressure or migraines, but may also interact with grapefruit or grapefruit juice. According to the Harvard Medical School Family Health Guide, when these medications interact with the fruit or juice, blood levels of the drugs are increased. Medications in this class that interact with grapefruit include felodipine -- brand name Plendil -- and nifedipine; calcium channel blockers that do not interact with grapefruit include verapamil, diltiazem and amlodipine, notes Harvard Medical School.

Statins

Statins are drugs used to lower high cholesterol levels. MayoClinic.com lists several statins that interact with grapefruit, including simvastatin, lovastatin and atorvastatin. If you eat a lot of grapefruit or drink a lot of grapefruit juice, you may want to ask your physician about statins that will not interact with the fruit, like fluvastatin, pravastatin, or rosuvastatil, notes Harvard Medical School.

Neurological and Psychological Medications

Various medications used for neurological and psychological conditions, like anxiety or depression, also interact with grapefruit. These medications include benzodiazepines like diazepam and triazolam and antidepressants like buspirone and sertraline, says the Harvard School of Medicine. Anti-seizure medications like carbamazepine can also be affected by grapefruit, adds MayoClinic.com. Medications that have little to no interaction with grapefruit include clonazepam, flurazepam, haloperidol and zolpidem.

Other Medications

MayoClinic.com lists other medications that may be affected by grapefruit, including the anti-arrhythmia drug amiodarone, the antihistamine fexofenadine, and several anti-retroviral drugs used to treat HIV and AIDS, including saquinavir and indinavir. If you are taking any medication not listed here, consult your health care provider to see if it may be affected by grapefruit or grapefruit juice.