I take a couple of medications that say “avoid grapefruit and grapefruit juice” but I have been unable to figure out exactly when grapefruit consumption is a problem. I assume eating a grapefruit everyday is not recommended - but what about 2 ounces of grapefruit juice in a cocktail or grapefruit flavored hard candy?
I know I should ask my doctor/pharmacist but I always forget so I thought I’d see if anyone here has any idea.
I don’t know the answer to the OP, but I would guess that it’s dependent on the medication. From what I understand, GF can slow the rate at which meds are metabolized. That means that there will be more still in your system by the time of your next dose. Whether an excessive amount is a problem will likely depend on the mediation. Some meds are no big deal to have a bit more, while others can be toxic. Also, I would guess that of the type of meds affected by GF, different meds would be affected differently. Some meds might still be mostly metabolized, while others may have a significant amount not yet metabolized.
One whole grapefruit or 200 mL of grapefruit juice is sufficient to cause clinically relevant increased systemic drug concentration and subsequent adverse effects. Seville oranges, (often used in marmalades), limes and pomelos also produce this interaction. Varieties of sweet orange, such as navel or valencia, do not contain furanocoumarins and do not produce this interaction.
SOURCE (everything you’ve ever wanted to know about the subject)
Thanks!
Note that grapefruit interfere at the intestine and at the liver, with typical major interaction at the intestine.
That means that taking medication with grapefruit can have a major effect. Typically, taking grapefruit between doses may have little or no effect.
Don’t wash down your grapefruit-sensitive medication with grapefruit juice, or with a spoonful of marmalade.
It’s been explained to me (perhaps incorrectly) that grapefruit INCREASES the amount of statins in my body, in which case I need to inquire: so why don’t I take a lower dose of statin with a regularly prescribed dosage of grapefruit?
I’d read that grapefruit makes your body destroy less of certain medications, effectively upping the dose. I know from personal experience that I can eat half a grapefruit without it interfering too much with my bupropion. This past Christmas, my sister brought our mother’s ambrosia (sliced oranges and grapefruit with cherries and fresh coconut slivers). I had a bowl, and drank down the juice. It was lovely… until two hours later when I could not keep still and was a Chatty Cathy - two things that are not conducive to a good yoga practice.
Here’s a handy summary from the FDA
TLDR: systemic levels of most drugs can be increased by grapefruit (etc); but levels of some drugs can be reduced. If you are taking several drugs which are affected by grapefruit, then that’s potentially complicated.
j
I never liked grapefruit or grapefruit juice. Once I started taking Atorvostatin, I was delighted to have an excuse not to consume either. 
I love a big Texas ruby grapefruit, and I used to eat them regularly. However, I haven’t tasted one in years, ever since I went on Simvastatin. I miss them, and wish they’d develop a variety which didn’t interfere with statin drugs.
Worth noting here that rosuvastatin (Crestor), pravastatin, or pitavastatin (Livalo, Zypitamag) aren’t known to interact with grapefruit.