No, I never had any negative experiences with cumin. It just did its job reliably. Good cumin! Well done! <pet, pet>
I think that's along the lines of what I experienced when I forgot to take it, got PEM, and then took the cumin. For me a dose consistently lasted 3 days; I never had it last only a few hours. Individual variation I suppose.
I still don't have a solid hypothesis for how it worked. Since it worked when absorbed sublingually and not swallowed, I'm reasonably convinced that its effect is on brain cells. This paper (
https://www.ncbi.nlm.nih.gov/labs/pmc/articles/PMC7880363/ ) does show that cuminaldehyde does affect gene expression in brain cells. Research has also found that it has other effects. I occasionally google 'cuminaldehyde' to look for new research, but I haven't yet found any obvious 'this is the answer'. I'm not enough of a biologist to have a good feel for how quickly the various body processes occur. I didn't try cumin in capsules, so I don't know whether having it bypass sublingual absorption would have added a delay.
I too worried about taking cumin--or anything else--too much too frequently. The first time I noticed cumin having an effect on me was before I even knew about ME. A tsp of cumin gave me temporary remission the first few times, but after a week or two I had to accept that it no longer had an effect. I did wonder if I had overdone it, but now it seems more likely that it's just ME's response to many (most?) treatments, and it would have lost effectiveness even if I'd used the bare minimum. I do try to find the minimum effective dosage for any treatment just to avoid possible downsides. For me, a level tsp every 3 days worked well, without problems.
All I can recommend is to try different amounts to see what is the minimum effective dosage for you, and how long it lasts. I suppose you can also experiment to see if there are any cofactors that might affect the dosage. Maybe it's better to take it with carbs, or oil, or rest or exercise. Consider it exploring uncharted waters. I explored, and found a true treasure. Most sail around not finding anything. A few got seasick (hating the taste).
I wish you good luck in your explorations, and I hope you post your results.