Few allergic symptoms have been reported with saw palmetto. A study of people taking the combination product PC-SPES® (no longer commercially available), which includes saw palmetto and seven other herbs, reports that three out of 70 people developed allergic reactions. In one case, the reaction included throat swelling and difficulty breathing.

Few severe side effects of saw palmetto are noted in the published scientific literature. The most common complaints involve the stomach and intestines, and include stomach pain, nausea, vomiting, bad breath, constipation, and diarrhea. Stomach upset caused by saw palmetto may be reduced by taking it with food. Some reports suggest that there may be less abdominal discomfort with the preparation lipidosterolic extract of Serenoa repens (LSESR). A small number of reports describe ulcers or liver damage and yellowing of the skin (jaundice), but the role of saw palmetto is not clear in these cases. Similarly, reports of headache, dizziness, insomnia, depression, breathing difficulties, muscle pain, high blood pressure, chest pain, abnormal heart rhythm, and heart disease have been reported, but are not clearly caused by saw palmetto. People with health conditions involving the stomach, liver, heart, or lungs should use caution.

Caution is advised in people scheduled to undergo some surgeries or dental work, who have bleeding disorders, or who are taking drugs that may increase the risk of bleeding. Dosing adjustments may be necessary.

Some men using saw palmetto report difficulty with erections, testicular discomfort, breast tenderness or enlargement, and changes in sexual desire. Saw palmetto may have effects on the body's response to the sex hormones estrogen and testosterone, but no specific effect has been well demonstrated in humans. Men or women taking hormonal medications (such as finasteride/Proscar®/Propecia® or birth control pills) or who have hormone-sensitive conditions should use caution. Tinctures may contain high levels of alcohol and should be avoided when driving or operating heavy machinery.

In theory, PSA (prostate specific antigen) levels may be artificially lowered by saw palmetto, based on a proposed mechanism of action of saw palmetto (inhibition of 5-α-reductase). Therefore, there may be a delay in diagnosis of prostate cancer or interference with following PSA levels during treatment or monitoring in men with known prostate cancer.

The combination product PC-SPES®, which contains saw palmetto and seven other herbs, has been found to contain prescription drugs including warfarin, a blood thinner. The U.S. Food and Drug Administration (FDA) has issued a warning not to use PC-SPES® for this reason, and it is no longer commercially available.

Because of possible hormonal activity, saw palmetto extract is not recommended for women who are pregnant or breastfeeding. Many tinctures contain high levels of alcohol and should be avoided during pregnancy.

For enlarged prostate (benign prostatic hypertrophy), a dose of 320 milligrams daily, in one dose or two divided doses (80% to 90% liposterolic content), has been used in numerous studies. Reports suggest that 160 milligrams once daily may be as effective as twice daily.

Traditional or other suggested doses that are less studied include: 1 to 2 grams of ground, dried, or whole berries daily; 2 to 4 milliliters of tincture (1:4) three times daily; 1 to 2 milliliters fluid extract of berry pulp (1:1) three times daily; or tea (2 teaspoons dried berry with 24 ounces water, simmered slowly until liquid is reduced by half) taken as 4 ounces three times daily. Teas prepared from saw palmetto berries are potentially not as effective because the active ingredients may not dissolve in water. Some experts believe that a preparation called lipidosterolic extract of Serenoa repens (LSESR) may cause fewer side effects.

Not enough information is available to recommend the use of saw palmetto in children.

Saw palmetto may increase the risk of bleeding when taken with drugs that increase the risk of bleeding. Some examples include aspirin, anticoagulants ("blood thinners") such as warfarin (Coumadin®) or heparin, anti-platelet drugs such as clopidogrel (Plavix®), and non-steroidal anti-inflammatory drugs such as ibuprofen (Motrin®, Advil®) or naproxen (Naprosyn®, Aleve®). Some batches of the discontinued combination herbal preparation PC-SPES®, which contains saw palmetto and seven other herbs, has been found to contain several medications including the "blood thinner" warfarin.

Saw palmetto should not be taken with drugs that affect the levels of male sex hormones (androgens), such as finasteride (Proscar®, Propecia®) or flutamide (Eulexin®). In theory, saw palmetto may interfere with birth control pills or hormone replacement therapy in women. Tinctures may contain high levels of alcohol and may cause nausea or vomiting when taken with metronidazole (Flagyl®) or disulfiram (Antabuse®).

Study in normal volunteers reveals no effects of saw palmetto on cytochrome P450 3A4 or 2D6 activity.

Based on at least two reports of serious bleeding, saw palmetto may increase the risk of bleeding when taken with herbs and supplements that are believed to increase the risk of bleeding. Multiple cases of bleeding have been reported with the use of Ginkgo biloba, and fewer cases with garlic. Numerous other agents may theoretically increase the risk of bleeding, although this has not been proven in most cases.

Because saw palmetto may have activity on the body's response to estrogen, the effects of other agents believed to have estrogen-like properties may be altered.

Tannins in saw palmetto may prevent iron absorption.