Research review
Saw palmetto: what the research does and doesn't show
A round-up of saw palmetto questions that don't have their own dedicated review: whether it causes constipation, diarrhea, weight gain, headaches or joint pain, whether it's safe in pregnancy, and whether it affects mood. On most of these, the honest answer is thin or absent evidence, not a confirmed effect in either direction.
The evidence doesn't support a confirmed answer, in either direction, for most of what's asked about saw palmetto beyond the prostate, hair and safety questions covered elsewhere on this site: constipation, weight gain, joint pain, mood, and pregnancy safety either weren't measured in the available research or weren't among the commonly reported effects in the largest safety review.
What the evidence doesn't support
It doesn't support saw palmetto causing constipation or weight gain — neither shows up among the commonly reported effects in the largest systematic safety review. It doesn't support any claim about mood, positive or negative, since no trial measured it as an outcome. And it doesn't support use in pregnancy, where the honest answer is an absence of research rather than a demonstrated safety record.
What the trials found
A systematic review pooling 40 studies on saw palmetto's adverse events found its side effects generally mild and similar in frequency to placebo. The effects it lists as commonly reported are abdominal pain, diarrhea, nausea, fatigue, headache, decreased libido and rhinitis. Constipation and weight gain are not among that list — which means the available safety literature doesn't report them as a typical effect, though the absence of a report in a review isn't the same as a trial that specifically looked for either and found none. Headache, by contrast, is directly reported as a commonly occurring side effect in that same review.
Body weight was checked directly in one controlled rat study, though as a safety marker, not a primary outcome: it found no significant difference in body weight between saw palmetto-treated and control rats at 2x and 5x the human recommended dose, over 2 to 4 weeks. That's animal data, not a human weight study, but it's a real data point against a weight-gain effect at those doses. See does saw palmetto help with weight loss or obesity? for the fuller picture on weight.
No trial we found tested saw palmetto for joint pain, either as a side effect or an outcome, and none measured mood using a validated scale or questionnaire. On pregnancy specifically, no dosing or safety trial has been conducted in pregnant or nursing people, which is why it's treated as an avoid-or-ask-a-doctor situation in who should not take saw palmetto, not because of a documented harm, but because of an absence of any research to say otherwise.
Questions people ask
Does saw palmetto cause constipation?
A systematic review of 40 studies lists diarrhea, not constipation, among saw palmetto's commonly reported side effects. Constipation wasn't reported as a common effect in that review.
Does saw palmetto cause diarrhea?
Yes, it's among the commonly reported side effects in the largest systematic review of saw palmetto's adverse events, alongside abdominal pain and nausea — though the review found these effects mild overall and similar in frequency to placebo.
Does saw palmetto cause weight gain?
It's not listed among the commonly reported side effects in the largest systematic review. A separate rat safety study found no effect on body weight at up to 5 times the human recommended dose, though that's animal, not human, data.
Can saw palmetto cause headaches?
Headache is listed among the commonly reported side effects in a systematic review of 40 studies, alongside stomach upset, fatigue and decreased libido — reported as generally mild and similar in frequency to placebo.
Can saw palmetto cause joint pain?
No trial we found tested for this specifically or reported it as a side effect.
Is saw palmetto safe during pregnancy?
No dosing or safety research exists for saw palmetto in pregnancy. It should be avoided, or only used after talking with a doctor, given that absence of data rather than any documented safety record.
Can saw palmetto help with mood?
No trial has measured mood as an outcome of taking saw palmetto, positive or negative.
When to talk to a doctor
Saw palmetto is not a treatment for any of the symptoms above. Persistent constipation, diarrhea, joint pain, or a mood change are worth discussing with a doctor to rule out other causes, rather than attributing them to or treating them with a supplement that hasn't been studied for any of them. If you're pregnant or nursing, ask a doctor before taking saw palmetto at all.
Related reading: saw palmetto side effects, who should not take saw palmetto, and is it safe to take saw palmetto every day.
Sources
- Serenoa repens (saw palmetto): a systematic review of adverse events A systematic review of 40 studies found saw palmetto's side effects are usually mild and similar in frequency to placebo — mainly stomach upset, headache, fatigue and decreased libido — with serious events reported only in isolated case reports of uncertain cause.
- Hepatotoxicity potential of saw palmetto (Serenoa repens) in rats A controlled rat study found no measurable liver-toxicity markers at doses 2 to 5 times the maximum recommended human daily dose, over 2 to 4 weeks — an animal safety finding, not a human one.
- Intraoperative haemorrhage associated with the use of extract of Saw Palmetto herb: a case report and review of literature A case report described one patient on saw palmetto with severe bleeding during surgery and a prolonged bleeding time that returned to normal after stopping the herb — a single case, not proof of cause, but a reason to disclose any supplement use before a procedure.
*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.