Grapefruit: why this class is not the class that cares Writing it up because I had to work it out twice and would rather nobody else did.
I was wrong about grapefruit in a thread last spring and I would like to correct it publicly rather than quietly.
The error was in the units, which changed the conclusion by an order of magnitude. Setting out the corrected version, and the way I now check for that class of mistake.
Adding the measurement that the opening post says would settle it.
Anticoagulants: no direct interaction with the compounds in this class. Weight loss and body composition changes might affect the clearance or effect of warfarin if you are on it; monitoring INR more frequently during weight loss is reasonable.
For anyone finding this later: the short answer on grapefruit is that it depends on one thing, and the rest of the thread is people identifying which thing.
Where a published interaction study exists it usually reports an area-under-curve ratio, and that number is far more informative than a yes-or-no answer.
I have seen it go both ways, which is why I hedge.
On the opening post — agreed on the reasoning, with one qualification.
Speaking only to grapefruit as I have actually seen it, rather than as it is usually described: the effect is real, it is smaller than the thread suggests, and the variance between people is larger than the effect.
A pharmacist can answer most questions in this subcategory in a few minutes with access to a proper interaction database, and that access is the thing a forum does not have.
Someone will know this better than I do and I hope they say so.
A request rather than an answer: could whoever has the primary source for grapefruit post it? I have seen the claim three times this month and each version had lost a qualifier.
Two claims get bundled together under grapefruit and they need separating. The descriptive one — this is what was observed — is usually well supported. The causal one — this is why — usually is not.
Almost every disagreement in threads like this one dissolves once you say which of the two you are making.
Building on post #10 rather than restating it.
The most defensible general position in this subcategory: identify the plausible mechanism, check whether it has been studied, and where it has not, say that rather than filling the gap.
Reading this grapefruit thread as someone who came in with a fixed view: the third and seventh replies moved me and the confident ones did not.
Taking post #14 at face value and following it one step further.
On grapefruit, I would rather understate and be corrected upward than overstate and be quoted. That is a house style here and it is a good one.
Post #18 answers the question as asked. The question underneath it is different.
Insulin and sulfonylureas are the interaction that the labelling in this class flags most explicitly, because the risk is additive glucose lowering. That is a prescribing question and not a forum question.
On grapefruit I would separate what is worth knowing from what is worth acting on. The first list is long and the second is short, and conflating them is how threads get heated.
Coming back to post #21, because the follow-up matters more than the original answer.
The honest answer on grapefruit is that it depends, and the useful part is the list of what it depends on. Four items, in rough order of how much they matter.
Most people get the first two right and then argue about the fourth.
Post #21 is right about the mechanism and I think understates the practical bit.
Where a published interaction study exists it usually reports an area-under-curve ratio, and that number is far more informative than a yes-or-no answer.
Someone should write this up properly, and it should probably not be me.
I read post #25 twice before replying, because I had assumed the opposite.
Oral contraceptive absorption has been studied specifically for some compounds in this class and the findings are compound-specific. Generalising from one to another is not supported.
I keep a log of this specifically because memory is unreliable about it.
Fine by me. I had wanted a stronger conclusion and there is not one available.
Adding the measurement that post #29 says would settle it.
Grapefruit is a question about a distribution, not about a value, and treating it as a value is what produces the confident wrong answers.