How does grapefruit ( juice ) affect medicines?

Grapefruit (juice) contains some natural substances such as furanocoumarin that can inhibit the function of Cytochrome P-450 3A4 enzyme in our small intestines. Some medicines normally broken down by such enzyme may decrease their metabolisms when taken with grapefruit (juice). Therefore, the drug concentration in the blood is increased. Excess medicine in our body may worsen the serious adverse reactions.

Which medicines should NOT be taken with grapefruit (juice)?

Medicines that grapefruit ( juice ) can inhibit their normal metabolisms  include:

• Antihistamines: Astemizole, Terfenadine

Antihistamine is often used to release symptoms from allergic rhinitis, runny nose, and scratch. The above two antihistamines are subject to the effect of grapefruit ( juice ) and may increase the risk of adverse reaction on the heart. Hence, grapefruit (juice) is prohibited while taking these medications.

( CMUH currently does not carry these two antihistamines )

• Antihypertensive drugs: Dihydropyridine category such as Amlodipine, Felodipine, Lacidipine, Nifedipine OROS, and Verapamil

The combination with grapefruit (juice) could lead to headache, hot flushes, and serious hypotension.

• Antihyperlipidemics: HMG-CoA reductase inhibitor (Statin category) such as Atorvastatin and Simvastatin

The combination with grapefruit ( juice ) could lead to serious muscle ache and muscular lesion. 

• Combination medicines : Exforge (Amlodipine and Valsartan), Vytorin (Simvastatin and Ezetimibe)
• Antiarrhythmic drugs : Amiodarone

The combination with grapefruit  (juice ) could increase the blood concentration of the medicine and affect the control over heart beats.

• Immunosuppressants : Cyclosporin, Tacrolimus

The combination with grapefruit ( juice ) could increase the blood concentration of the medicine and lead to vomiting, headache, light feet, abdominal pain, and even kidney toxicity.

• Anticonvulsants: Carbamazepine

The combination with grapefruit ( juice ) could increase the blood concentration of medicine and lead to drug poisoning. 

What are the precautions for medication safety?

• It is recommended to take medicines with plain water to avoid the adverse reactions caused by drug interactions. Please do not take medicines with drinks, such as grapefruit ( juice ), cola, tea, or milk, unless you are specially instructed.
• If you have been taking medicines with grapefruit ( juice ), please do not abruptly change the habit by suspending the drink immediately. It is advised to consult physicians or pharmacists to avoid the sudden drop of drug concentration in the blood.
• If you do not know if the medicines you are currently taking interact with grapefruit ( juice ), please consult your physicians or pharmacists to ensure your medication safety.