Perimenopause: What to Test Before You Supplement
THE SHORT ANSWER
Perimenopause changes what your body needs, but not in the way most supplement marketing implies. Three things are worth acting on and one of them is a blood test, not a bottle. Ferritin, because heavier or more frequent bleeding depletes iron stores and the fatigue trials were run in exactly this population. Vitamin D, because bone loss accelerates and status varies too much between people to guess. Magnesium, because it has the best evidence of anything on the shelf for the sleep disruption that defines this stage. Almost everything else marketed for perimenopause is weaker than it sounds.*
011. Test ferritin before anything else
This is the single highest-value thing in this article and it costs a blood draw.
Perimenopause frequently brings heavier and less predictable bleeding, and iron stores fall accordingly. The important part is that you can be iron deficient without being anaemic — haemoglobin looks normal while ferritin is depleted — and that is precisely the population the fatigue research studied.
A 2003 double-blind randomised trial gave 144 non-anaemic women with unexplained fatigue 80 mg of elemental iron daily; fatigue fell relative to placebo, concentrated in those with low ferritin.1 A 2012 randomised trial in menstruating women with low ferritin found the same.2
So: ask for ferritin, not just a full blood count. If you are low, Iron Complex or Iron Bisglycinate. If you are not, do not take iron — it accumulates and there is no efficient way to shed it. The marker most people guess at covers what to ask for.
022. Vitamin D — measure, do not assume
Bone loss accelerates through the perimenopausal transition, which makes vitamin D status matter more than it did at thirty. But status depends on latitude, season, skin tone and sun exposure, and those vary so much between individuals that a single blanket recommendation is close to meaningless.
The strongest pooled evidence for vitamin D supplementation shows benefit concentrated in those who were deficient to begin with.3 Which is the same pattern as iron, and the same argument for testing.
If you do supplement, D3 travels with K2 — the reasoning, and the product. The fuller argument for testing is in the question a blood test answers better than a label.
033. Magnesium, for the sleep
Sleep disruption is one of the most consistently reported features of this stage, and magnesium has better evidence than anything else sold for it.
A 2025 randomised placebo-controlled trial gave adults with poor sleep 250 mg of elemental magnesium as bisglycinate. Insomnia severity fell 3.9 points against 2.3 for placebo, with improvement greater in those whose dietary magnesium was low.4 We will say again what we said in the sleep article: the authors describe the effect as small, and the p-value was 0.049.
It is modest, well tolerated and cheap. That combination is rarer in this category than it should be. Glycinate is the form with the trial on the form; which magnesium should you take compares all four.
04What we are not going to tell you
That a capsule replaces hormone therapy. It does not, and the comparison is not close. If your symptoms are significantly affecting your life, that is a conversation with a doctor about HRT, and a supplement company is not the right source of advice on it.
We are also not going to make claims about hot flushes. You will find plenty of pages that do. The evidence for supplements on vasomotor symptoms specifically is weaker and more mixed than those pages suggest, and we would rather leave a gap than fill it with something we cannot cite.
05The things that are probably worth it, honestly labelled
Active folate. Heavier bleeding affects folate as well as iron, and a meaningful share of people convert folic acid inefficiently. 5-MTHF is the active form — why that matters.
Inositol, if insulin resistance is part of your picture. The 40:1 ratio is the studied one — why the ratio matters. Here.
L-theanine, if the problem is a mind that will not switch off rather than a body that will not settle. Here.
06What outperforms all of it
Resistance training. It acts on bone density, lean mass, insulin sensitivity and sleep simultaneously, and no capsule on this page does that. We are aware of how this reads coming from us. It is still the honest ordering.
The Women's Health range is here when you want it.
07What we can actually show you
Everything above is about nutrients and their evidence, which is modest and conditional. What is not conditional is what is in the bottle: every lot independently tested for identity, potency, heavy metals and microbials, with the certificate for the lot currently shipping published on the product page.
08Frequently asked questions
Which blood tests should I ask for?
Ferritin and vitamin D are the two where the result genuinely changes what you should do. A full blood count alone can look normal while ferritin is depleted.
Should I take iron for perimenopausal fatigue?
Only if ferritin is low. The trials showing benefit were run specifically in women with low ferritin.1,2 Taking it without testing carries real downside.
Do supplements help hot flushes?
We are not going to claim they do. The evidence on vasomotor symptoms is weaker and more mixed than most pages selling them admit.
Can supplements replace HRT?
No. If symptoms are significantly affecting your life, that is a medical conversation, not a supplement one.
When does perimenopause start?
It varies widely, commonly beginning in the forties and sometimes the late thirties, and can run for several years before periods stop.
How long before I notice anything?
The trials referenced here ran four weeks for magnesium and four weeks for iron.1,4 Judge over weeks, and only after you have tested.
09References
- Verdon F, Burnand B, Fallab Stubi CL, et al. Iron supplementation for unexplained fatigue in non-anaemic women: double blind randomised placebo controlled trial. BMJ. 2003;326(7399):1124. doi:10.1136/bmj.326.7399.1124
- Vaucher P, Druais PL, Waldvogel S, Favrat B. Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin: a randomized controlled trial. CMAJ. 2012;184(11):1247–1254.
- Martineau AR, Jolliffe DA, Hooper RL, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017;356:i6583.
- Magnesium bisglycinate supplementation in healthy adults reporting poor sleep: a randomized, placebo-controlled trial. 2025. PMCID: PMC12412596.
- Chan V, Lo K. Efficacy of dietary supplements on improving sleep quality: a systematic review and meta-analysis. Postgrad Med J. 2022;98(1158):285–293. doi:10.1136/postgradmedj-2020-139319
*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. This article is not medical advice; speak to your doctor about symptoms and about hormone therapy.
