DIM for Estrogen Dominance: Does It Work, and Is It Safe?

What is DIM?

DIM (3,3'-diindolylmethane) forms in the stomach when you eat cruciferous vegetables such as broccoli, cabbage, cauliflower, kale and Brussels sprouts. These contain glucobrassicin, which breaks down into indole-3-carbinol (I3C), and stomach acid converts I3C into DIM. As a supplement, DIM is sold in capsules, often in an "absorption-enhanced" form, and marketed for estrogen dominance, hormonal acne, PMS and weight loss.

DIM is a dietary supplement, not a medicine. In the US, supplements don't need FDA approval for safety or effectiveness before they are sold, and their contents can vary between brands.

How does DIM affect estrogen?

After estrogen has done its work, the liver converts it into metabolites. Two pathways get most of the attention: 2-hydroxylation, which produces weak metabolites, and 16-alpha-hydroxylation, which produces a metabolite with stronger estrogen-like activity. DIM appears to nudge metabolism toward the 2-hydroxy pathway, raising the "2:16 ratio" measured in urine.

The human evidence for that effect comes from small studies:

• In a 30-day pilot trial of 19 postmenopausal women with a history of early breast cancer, 108 mg a day of DIM increased urinary 2-hydroxyestrone compared with placebo (Dalessandri 2004).
• In a 14-day pilot study in patients with thyroid nodular disease, DIM was detected in thyroid tissue and raised the urinary 2:16 ratio (Rajoria 2011).
• In the largest randomized trial, 130 women taking tamoxifen after breast cancer were assigned DIM (150 mg twice daily) or placebo for 12 months, and 98 completed it. DIM increased the 2:16 ratio and raised SHBG, the protein that binds estrogen. It did not change breast density (Thomson 2017).

Eating more cruciferous vegetables has a similar, smaller effect on the 2:16 ratio (Fowke 2000).

Does DIM actually work for estrogen dominance?

That depends on what you mean by "work." DIM reliably changes lab measures of estrogen metabolism. What hasn't been shown in clinical trials is that it improves the symptoms people take it for: heavy periods, PMS, breast tenderness, bloating, acne, mood or weight.

It's also unclear how much the 2:16 ratio matters. Early studies of the ratio and breast cancer risk gave inconsistent and mostly non-significant results, partly because older assays weren't precise. Newer, more accurate methods suggest more 2-hydroxylation is associated with lower postmenopausal breast cancer risk (Ziegler 2015). But an association in population studies doesn't prove that raising the ratio with a supplement lowers risk, and no trial has shown that.

"Estrogen dominance" itself isn't a formal diagnosis, and its symptoms have many causes; see what estrogen dominance really means.

Is DIM safe?

Side effects

In a single-dose study of absorption-enhanced DIM, no side effects were reported up to 200 mg; at 300 mg, one of six people had mild nausea and headache and one vomited (Reed 2008). In the 12-month tamoxifen trial, side effects were minimal and similar to placebo. People also report digestive upset, headache and darker urine. Long-term safety at supplement doses is not well studied.

Drug interactions

This is the bigger concern. In the tamoxifen trial, women taking DIM had significantly lower blood levels of tamoxifen's active metabolites, including endoxifen, and the researchers flagged that this might reduce tamoxifen's benefit (Thomson 2017). DIM and I3C also affect liver enzymes that process many medicines. Don't take DIM without checking with your clinician if you use:

• Tamoxifen, aromatase inhibitors or other cancer treatments
• Menopausal hormone therapy or hormonal birth control
• Thyroid medicine, blood thinners, or other regular prescriptions

Who should avoid DIM

DIM vs. I3C vs. calcium-D-glucarate

These three often appear together in "estrogen detox" products, but they're different:

• Indole-3-carbinol (I3C) is the parent compound from vegetables. It is unstable in stomach acid and turns into DIM and other products, so its effects are less predictable.
• DIM is the main product of I3C and the form most supplements now use.
• Calcium-D-glucarate is claimed to block beta-glucuronidase, a gut enzyme that can reactivate estrogen. Human evidence that it changes estrogen levels or symptoms is very limited.

Combination products also add herbs whose amounts and interactions are rarely tested together. More ingredients don't mean more benefit.

Can a test show whether DIM is working?

Food or supplement?

Cruciferous vegetables deliver the compounds that form DIM in smaller amounts, plus fiber and other nutrients, and they have no known drug interactions at normal intakes. For most people, eating them most days is a better first step than a capsule. If you choose a supplement, studies have used roughly 100 to 300 mg a day of an absorption-enhanced form; buy from brands that use third-party testing, and stop if you get side effects. See the estrogen dominance diet for food-based options and the estrobolome for the gut's role in estrogen.

What works better for common "estrogen dominance" symptoms?

• Heavy or irregular bleeding: evaluation for fibroids, polyps, thyroid disease and other causes, then treatments such as progestogens or a hormonal IUD
• Severe PMS or PMDD: SSRIs and some hormonal options have trial evidence
• Breast pain: a well-fitted bra, reducing caffeine if it seems to trigger it, and evaluation of any lump
• Higher estrogen from excess weight or alcohol: weight management and cutting back on drinking

For many women in their 40s, these symptoms are the early phase of perimenopause, and hot flashes, night sweats and poor sleep may follow as estrogen declines. At that point, menopausal hormone therapy, combining estrogen with a progestogen for anyone with a uterus, is the most effective treatment. Amie's licensed clinicians can review your history and, if appropriate, prescribe Amie HRT+, a compounded estradiol and progesterone cream. It is not FDA-approved, and the clinician decides whether it's suitable.

The bottom line