I have dealt with increasingly problematic IBS-C since a long round of oral antibiotics for Lyme ten years ago. Concurrent with this I've faced increasing fatigue type troubles - I eat, and 90 minutes later I am very likely going to lay down and there is no getting out of this. I knew my blood pressure was dropping during these episodes, but had no idea how dangerous it was.
I got the flu at the end of the year, and didn't eat or take any medication or supplements for 36 hours, I just literally laid in bed. When I came to around 1:00 AM on New Years Day I crawled to get gabapentin and kava kava in me. Half an hour later my blood pressure had come up ... to 81/62, and my pulse was still unmeasurable - my heart was basically aggressively fluttering rather than beating properly. I'm lucky to be alive.
I talked with a friend who was a med school prof and right away he said "paroxysmal atrial tachycardia" - that's a symptom, not a diagnosis, but it's one that can be treated aggressively. He asked if I had magnesium, and I did - I was take 200mg every morning of Doctor's Best High Absorption Magnesium. He told me take one every three hours until the problem went away.
And this recommendation has been plain magic.
I was on 300 mg gabapentin three times daily for chronic pain from a car crash, and I was doing some or all of the following for IBS-C and/or fatigue.
psyllium husk fiber
Since getting this recommendation I have taken an average of 900mg of magnesium daily. If I haven't had a BM in a day I will take two 200mg tablets at the same time. Five weeks have elapsed, during this time I have had ONE glass of prune juice and I've taken 45 kava kava, always in response to a heart rate spike/blood pressure drop after eating.
Having stopped all the IBS-C stuff is great, what is not quantifiable is the fact that I no longer feel like my head is full of fluff every day. I get up, I work for four or five hours, take a break, come back and do something else for a couple more hours. I am more productive in a day and a half now than I would have been in a whole week.
This doesn't cost a lot and unless your kidneys are below par there is no real hazard to going over the RDA. Be sure to get a chelate or citrate, the cheap magnesium oxide sold at the big name pharmacies is terrible for biological availability.