Saw palmetto is one of the most widely used herbal supplements, and its side effect profile is remarkably mild. Large clinical trials, including an 18-month NIH-funded study that tested doses up to 960 mg daily (three times the typical dose), found no statistically significant difference in adverse events between saw palmetto and placebo. That said, a few side effects do show up in practice, and certain groups of people should be cautious.
How Saw Palmetto Works in the Body
Understanding the mechanism helps explain why certain side effects occur. Saw palmetto blocks an enzyme that converts testosterone into a more potent hormone called DHT. It reduces DHT’s ability to bind to receptors by nearly 50% and also promotes the conversion of DHT into a weaker, less active form. This is why it’s commonly taken for enlarged prostate symptoms and hair loss. It’s also why most of its notable risks relate to hormone-sensitive situations.
Common Mild Side Effects
The most frequently reported side effects are digestive: nausea, stomach discomfort, diarrhea, and constipation. These tend to be mild and are more likely when you take saw palmetto on an empty stomach. Taking it with food usually helps.
Headache and dizziness have also been reported, though in controlled trials these occurred at similar rates in people taking a placebo. That pattern held even as doses were escalated from 320 mg to 640 mg to 960 mg daily over 18 months, with no evidence of a dose-response relationship for side effects. In other words, higher doses didn’t produce worse or more frequent problems.
Interactions With Medications
Saw palmetto can interact with a few categories of medication. The most important ones to know about:
- Blood thinners and antiplatelet drugs: Saw palmetto may increase the risk of bleeding when combined with these medications. If you take warfarin or similar drugs, this combination warrants a conversation with your pharmacist or prescriber.
- Hormone therapy and oral contraceptives: Because of its anti-androgenic properties, saw palmetto may reduce the effectiveness of estrogen-based therapies and birth control pills. Women using hormonal contraception should be aware of this potential interaction.
Bleeding Risk Before Surgery
Early case reports raised concerns that saw palmetto might increase surgical bleeding. However, two randomized controlled trials specifically examining bleeding during prostate surgery found no increased risk. A 2022 review in Baylor University Medical Center Proceedings classified saw palmetto’s bleeding risk as “none” based on the available evidence, putting it in the same category as fish oil and ginseng.
Despite this, many surgeons still recommend stopping all dietary supplements about two weeks before any scheduled surgery, simply as a general precaution. If you have a procedure coming up, let your surgical team know you’re taking it.
Liver Safety
Saw palmetto has been linked to rare cases of liver injury, but the connection remains uncertain. The NIH’s LiverTox database rates it as a “possible, rare cause of clinically apparent liver injury,” its lowest level of concern. In the handful of reported cases, symptoms appeared within one to two weeks of starting the supplement and resembled a mild viral hepatitis, with liver enzyme levels returning to normal within one to three months after stopping use.
No cases have led to liver failure, transplantation, or chronic liver disease. Long-term studies have not found any pattern of liver enzyme elevations during regular use. This is about as close to a clean bill of health as a supplement can get on the liver front, but if you develop yellowing of the skin, dark urine, or unusual fatigue while taking it, stopping the supplement is a reasonable first step.
PSA Levels and Prostate Cancer Screening
One concern that comes up often is whether saw palmetto could mask prostate cancer by lowering PSA levels, since prescription drugs that work through a similar mechanism do exactly that. The good news: saw palmetto does not appear to affect PSA readings, even at higher-than-usual doses. You don’t need to stop taking it before a PSA test, and it shouldn’t interfere with prostate cancer screening.
Who Should Avoid Saw Palmetto
Because saw palmetto alters hormone activity, pregnant and breastfeeding women should not take it. Its anti-androgenic effects could theoretically interfere with fetal development, and there’s no safety data in these populations.
Women on hormonal birth control or hormone replacement therapy should also use caution, since saw palmetto may reduce the effectiveness of these treatments. And anyone on blood-thinning medication should discuss the supplement with their healthcare provider before starting it, even though the clinical evidence for bleeding risk is limited.
For most adults, saw palmetto at the standard 320 mg daily dose has a safety profile that holds up well in rigorous testing. The NIH-funded CAMUS trial concluded that for periods up to 18 months, there are no serious safety concerns associated with its use, even at nearly 1,000 mg per day. The side effects that do occur are typically mild, digestive, and manageable.