Nine weeks on fezolinetant, the hot flashes are genuinely better. But constipation started week 2 and hasn’t let up. The part that makes this hard to untangle: I’m also post-RYGB (2017) and eight months into tirzepatide.
Three separate mechanisms all slowing transit at the same time - altered bariatric gut anatomy, GLP-1 gastric emptying delay, and now fezolinetant on top. The standard “drink more water” advice doesn’t really account for the fact that tirz already blunts thirst signals, and post-RYGB absorption is just different. What’s actually moving the needle: 400mg magnesium glycinate at night, spreading fiber across the day rather than one big dose (dumping risk if I overload at once), and being deliberate about hydration timing around meals. For people without the bariatric history - does the constipation tend to resolve on its own after a few weeks, or does it just become the new baseline? Genuinely curious whether this is a timing issue or a “manage it indefinitely” situation.
the magnesium absorption piece post-RYGB is worth flagging, since most Mg gets absorbed in the duodenum and proximal jejunum, which is exactly the segment that’s bypassed. glycinate is more bioavailable than oxide so you’re ahead of the curve already, but a 400mg label dose isn’t 400mg of usable Mg in a bypassed gut, and serum Mg is a notoriously poor proxy for tissue Mg, the kidney compensates by holding onto it so the number looks fine while you’re functionally running low. RBC magnesium or a magnesium loading test are both better signals than serum, though neither is standard and you’d have to ask. the reason this matters for transit specifically is that low intracellular Mg impacts smooth muscle relaxation in the gut, so a functionally depleted state can make the supplement do less than the dose suggests. the other angle nobody’s mentioned: fezolinetant works on KNDy neurons in the hypothalamus, it suppresses the thermoregulatory signal but doesn’t touch the gut motility effects of estrogen loss, which are real and documented in the menopause transit literature, transit time goes up post-menopause independent of any drug. so the three mechanisms you named are real but there may be a fourth running underneath, estrogen withdrawal on smooth muscle and the enteric nervous system. that one doesn’t resolve on a timeline, it’s the new baseline unless someone’s on HRT, which fezolinetant isn’t. re your actual question, the GLP-1 constipation curve does tend to flatten for most people by month 3-4 ime, but with the RYGB confound stacked underneath, that’s a different intervention mechanistically than starting a GLP-1 in an unoperated gut, and i’m not sure the general timeline maps cleanly onto your situation. “resolves vs new baseline” might be the wrong frame, more like which mechanism is the rate-limiting one once you adapt to the others. SIBO is also worth ruling out fwiw, post-RYGB rates are non-trivial and it can present as alternating constipation rather than the textbook diarrhea picture. clinician conversation more than a forum one though.
The case for RBC Mg over serum is solid. Getting a GP to actually order it is another matter entirely, and serum is realistically what most of us receive.
the access barrier piece holds, RBC Mg is hard to get ordered without specifically asking and explaining why. the part i’d refine is framing serum as “realistically what we receive,” because serum Mg is so tightly regulated by kidney handling and bone reservoir release that it can sit comfortably mid-range while intracellular stores are genuinely depleted. that’s not a noise property, it’s a homeostatic-buffering one, the marker is doing exactly what it’s designed to do, it just isn’t measuring what most of us want it to measure. RBC Mg is a real improvement on that, but it’s still a proxy for tissue Mg rather than the thing itself, erythrocyte Mg and muscle/nerve Mg don’t track perfectly, and the ionized or functional Mg assays that get closer to what actually matters are even rarer to get ordered. the more practical version of this, given the stacked context in your OP (RYGB malabsorption plus tirz-driven intake reduction plus fezolinetant transit), is that empiric supplementation often outruns the diagnostic chase. proximal jejunum is where most Mg gets absorbed and that’s exactly the segment RYGB bypasses, so the prior probability of subclinical depletion is high enough that the supplementation decision often gets made regardless of what the serum number says. the test becomes more useful as a trend tool over time than as a single-draw yes/no on whether to treat. worth being honest that if serum is all you can get, the result is closer to “doesn’t tell you much in either direction” than “tells you you’re fine,” which is a different framing than the one that usually gets handed back with the lab. the value of pushing for RBC isn’t that it answers the question cleanly, it’s that it shifts the marker from one that’s been actively buffered to one that’s been less aggressively buffered, which is a real upgrade even if it isn’t the endpoint.