Is it true that grapefruit juice shouldn’t be consumed with medication because the acid can dangerously change the medication’s effect in your body?
If so, does this mean things like vinegar and citrus-fruit juice are dangerous alongside of medication too?
No, what’s true is that chemical compounds in grapefruit juice interacts with some medications and can dangerously change the effect. But it’s nothing to do with the acidity.
Any medication you swallow goes via the stomach, where the body continuously adds very strong acid. If it was just a question of acidity that would swamp out any effects from acidic foods.
As naita said, it’s not because of the acidity. Here’s a link.
Grapefruit juice needs an enzyme that’s made by your liver to metabolize it so your body can use it. Many medications need this same enzyme to metabolize them so your body can use them. There’s only so much of that enzyme to go around. So if you drink grapefruit juice and it uses up all of the enzyme your liver has made today, then there’s not enough of that enzyme left to metabolize your medicine. The medicine will sit there in its unmetabolized form and not its active, metabolized form. So it won’t do anything for you, and it might (depending on what it is) even harm you because it isn’t being metabolized like it was designed to.
It’s not every medication, but it’s a lot of them. That particular enzyme is a popular one.
There are some herbs that have the same problem, including St. John’s Wort and, of all things, black pepper. So before taking medicinal doses of anything, make sure you’ve checked with an herbalist or pharmacist that your drugs aren’t interfering with your herbs.
What I’ve always read is sort of the opposite of this. The drug remains unmetabolized in your body, but according to what I’ve usually seen, it’s the active form, not an inactive form. As you continue to take additional doses, the drug builds up in your system until you start to suffer from overdose effects. The above-linked Wiki page seems to agree with this.
Now, my question: Why do such discussions always always always mention “Grapefruit juice” and not just “Grapefruit”? 
If I just eat a whole raw grapefruit, does that make it okay then?
I used to eat whole raw grapefruit all the time, until I had to stop because of all the medicines I now have that say I can’t have grapefruit juice. But all those grapefruits I used to eat were contaminated with juice so I had to quit.
Yeah, I didn’t word that well. Also, there’s another interaction, while it’s more rare, wherein a chemical in grapefruit juice blocks the production of a protein that’s needed for the absorption of a medication, and you’ll have lower absorption with grapefruit. Allegra is one of those; there may be others.
Because we think it takes a lot more than the juice in one grapefruit to use up all the liver enzyme. So a fruit is likely okay. A small glass of grapefruit juice is probably okay. But it’s often said as “no grapefruit juice” because:
Simpler and safer to just make grapefruit juice off limits. And many labels include grapefruit as well. http://www.mnn.com/sites/default/files/main_grapefruit_drug_interaction.jpg
There’s probably little harm in drinking grapefruit juice 8 hours or more from the time you take medications affected by GF juice. If you take most of your meds in the morning, feel free to enjoy a greyhound or two in the evening. IANAD, YMMV.
BTW, some meds are affected by orange juice instead of grapefruit juice, but as stated above it’s not the acididy that’s involved.
Some drugs are affected by acidity (i.e. low pH). Those are usually the ones that are enteric-coated, or in some cases the vehicle is itself an antacid tablet. The first one that comes to mind is ddI, an AIDS drug that I’m not sure is even on the market any more.
Grapefruit juice is metabolized by the same liver enzyme as some drugs, and the other posters who said that a small amount of grapefruit itself is probably not harmful are probably right. It’s specifically the juice.
As others have said, it has nothing to do with acidity. Pretty much everything absorbed by the intestines into the bloodstream passes immediately through your liver, which contains a variety of enzymes (cytochromes) which “detoxify” any xenobiotics (eg drugs) by doing stuff like sticking hydroxyl groups on them to make them more polar and water-soluble, so you can pee them out.
This “first-pass” metabolism is of great importance in determining dosage of orally-administered drugs.
Grapefruits contain compounds which can enhance or reduce the activity of the cytochromes and hence the rate of metabolism of the drug, leading to blood levels that are too low (below the therapeutical level), or too high (potentially toxic, or enhanced side-effects). To be honest, I don’t remember if grapefruit induces or inhibits cytochrome activity, but you get the point.
What meds might these be? NEED ANSWER FAST.
The usual explanations I see are not that it enhances or inhibits cytochrome activity, but rather that the relevant chemical in the juice competes with the drugs for access to the cytochromes.
The same huge list as grapefruit, but for some reason (I don’t know why) we don’t see as many worries for other juices. Nonetheless:
http://www.pharmacist.com/juice-interactions-what-patients-need-know
That’s what I was taught, but now I’m seeing all the “inhibits” articles. Maybe they’ve learned something new. Darn science, keeps changing as more research is done. Most inconvenient. 
Reminds me of Mr. Weinstein, my 11th grade Algebra teacher. Whenever he (or sometimes a student) got caught making some mistake, he’d exclaim “Well darn, they’ve changed it again!”
He also explained the technique whereby the teacher could so often solve difficult problems that were obscure to the student: “It HELPS . . . if you’ve seen the problem before”. (I’ve used that line a few times myself in subsequent years.)
He also carefully explained to us that the correct pronunciation of 6! was not
SIX!!!ETA: Okay, back on topic: What’s a seville orange?
Ok it isn’t even grapefruit juice, but WHITE GFJ as far as I have seen that is the big issue. Furanocoumarins in the juice inhibit the CYP3A4 liver enzyme which metabolizes certain drugs. This can either make prodrugs not metabolize, or it can cause other drugs to hang around longer than usual.
Sometimes a drug is actually a prodrug, like loratadine or codeine where it needs to be metabolized to a secondary form to even work.
Thanks for handling this since Senegoid “NEED[ED] ANSWER FAST”. I guess needed to know if Ok to drink that pitcher of Mimosas. 
Study: Drinking Orange Juice Can Counteract Prescription Meds
Welcome to the wonderful emerging world of pharmacogenetics. It’s not just foods that can affect how your body processes drugs; it can also be your genetics. We don’t often think about how exactly our bodies manage to deal with all of these newly-developed chemicals and the chaotic chains of reactions they go through inside us. Any mutation in any of the genes responsible for any of the key reactions can strongly change what the drug does, either for good or ill.
I used to work in a lab that did some genetic tests for this sort of thing (among other things). There was one tragic case where we were sent samples from young twin boys who had died after being given Tylenol (with codeine, IIRC) at a hospital. It turned out that they both had a rare mutation in a key gene that caused the drug to be shunted into a side pathway that isn’t active in normal people. The end result was a buildup of a highly toxic end product that the body couldn’t deal with, and they both died.
Some classes of drugs, like those used to treat depression or other mental health issues, already recommend that patients be tested for certain mutations. It’s likely that in the future, your genetic profile will be used to figure out ahead of time which drugs will work best for you personally - that “personalized medicine” we’ve heard so much about.
Which is kinda sorta saying the same thing, but more scientific-like.
This thread suggests to me a few tangents:
Would it be possible to produce a cultivar of grapefruit (or other fruit, for that matter) that doesn’t have drug-interaction issues? Or would we then get too far afield, and the resulting fruit may be too dissimilar from the original?
Would it be possible to treat fruit juices after production to avoid drug-interaction issues? Compare milk treated with the enzyme lactase to aid digestion.
Would it be possible to take the proper enzymes orally soon after fruit-juice consumption so that the drug would work largely as expected? Or just take the enzyme with the drug so that there’s a guaranteed-sufficient amount of the enzyme available? Again, compare to lactase pills.
I wish they could develop a grapefruit that can be eaten safely by a person taking statin drugs. I take simvastatin, and I haven’t had a big juicy ruby grapefruit in years. I love them.
