I wrote on this board saying that this may have been placebo effect about 6 months ago. While I still think that most of the claims that celery seed makes such a big difference in only a few days are exactly that, I do think there may be something to it. After more than 6 months of taking it daily I have started noticing a more significant increase in volume for both cum and precum.
I haven't produced a large amount of precum since I was a teenager, but over the past few weeks I've been producing much more than I have in the past few years. While playing around, getting ready to masturbate but clothes still on I don't notice it, but once I take my clothes off I've noticed that I'm actually dripping. It surprised me at first when I got naked and whipped my cock around and a long stream of precum stretched from where it first landed on my stomach down to my thigh. Even after that I still had a lot of precum under my foreskin.
Ejaculation volume hasn't had such a large increase if I masturbate daily. But when I wait a couple of days between the amount of cum is greater than it was when I waited before taking celery seed regularly for as long as I have. My orgasms feel stronger and I cum with more force than previously.
I have been taking some other supplements in the meantime, fish oil and magnesium, but neither of these seem to be related. I've also started drinking a few tablespoons of apple cider vinegar diluted in water once a day, but I haven't heard any claims about that either. The only other change is a slight reduction in alcohol intake over the past couple of weeks, but these changes seemed to be present before that.
There might be more to it than a placebo effect. But I still think that any reaction that occurs in a couple of days is not the result of taking this supplement.