What do studies show about the use of testosterone in patients with breast cancer?

Testosterone Should NOT Be Used in Patients with Breast Cancer

Testosterone/androgen supplementation should not be used by men with breast cancer, and caution is strongly recommended for women with hormone-sensitive breast cancer. 1

Evidence from Guidelines

The 2020 ASCO guideline for male breast cancer explicitly states that testosterone/androgen supplementation should not be used by men with breast cancer (moderate strength recommendation, low evidence quality). 1 The rationale is clear: testosterone converts to estrogen through aromatization by the aromatase enzyme, raising significant concern about stimulating hormone receptor-positive breast cancer growth or recurrence. 1

For women, the 2019 Global Consensus Statement on testosterone therapy excluded women with prior breast cancer from randomized trials for hypoactive sexual desire disorder, and explicitly recommends caution for testosterone use in women with hormone-sensitive breast cancer. 2 The 2010 Endocrine Society guideline categorically lists metastatic prostate cancer and breast cancer as conditions where testosterone administration carries very high risk of serious adverse outcomes and should not be used. 3

The FDA testosterone label reinforces this, contraindicating testosterone in men with known or suspected breast or prostate cancer. 4

The Biological Concern

The mechanism of concern is straightforward: testosterone undergoes aromatization to estradiol, which can promote estrogen-dependent breast cancer progression. 1, 5 Early preclinical studies suggest high concentrations of exogenous testosterone may increase risk for estrogen-dependent cancer progression through this conversion pathway. 5 Additionally, androgen receptor activation by exogenous testosterone may directly promote breast cancer growth according to some preclinical studies. 5

Conflicting Research Evidence vs. Guideline Consensus

Here's where it gets nuanced: Several observational studies suggest testosterone therapy may actually reduce breast cancer incidence in women without prior breast cancer. Studies by Glaser et al. reported reduced breast cancer incidence in women treated with subcutaneous testosterone implants (142-165 cases per 100,000 person-years vs. 271-293 expected from SEER data). 6, 7, 8 A 2026 study showed similar findings (144 per 100,000 person-years). 6

One small 2014 study even reported therapeutic activity of testosterone propionate in 53 patients with metastatic breast cancer refractory to other hormonal therapies, showing 17% regression and 41.5% disease stabilization. 9

However, these research findings do NOT override guideline recommendations for several critical reasons:

• These are observational studies in women without existing breast cancer, not in breast cancer patients
• The studies lack long-term randomized controlled trial data
• The 2014 therapeutic study used testosterone propionate in heavily pretreated metastatic disease—a completely different clinical context
• Guidelines prioritize safety in the context of known hormone-sensitive disease

Clinical Management Algorithm

For Men with Breast Cancer:

1. Do not prescribe testosterone supplementation 1
2. If hypogonadism develops from endocrine therapy (tamoxifen, aromatase inhibitors with GnRH analogs):
   • Manage hot flashes with venlafaxine (shown effective in men with prostate cancer on GnRH analogs) 1
   • Address sexual dysfunction through non-hormonal approaches
   • Monitor bone density if on GnRH analog + aromatase inhibitor combinations 1
3. If a patient insists on testosterone for severe hypogonadism: This requires an informed discussion about potential benefits versus risks, considering residual breast cancer recurrence risk. The decision ultimately rests with a fully informed patient, but the default recommendation is avoidance. 1

For Women with Prior Breast Cancer:

1. Exercise extreme caution—women with prior breast cancer were excluded from testosterone trials 2
2. The 2025 guideline for transgender/gender-diverse patients notes that 35% would continue hormone therapy despite hormone-dependent cancer diagnosis, emphasizing the need for individualized, informed discussions 5
3. One 2024 pilot study (n=29) showed topical testosterone gel (3 mg/day) improved sexual function in premenopausal patients on ovarian suppression + aromatase inhibitors without elevating estradiol levels, but this is insufficient evidence for routine use 10
4. A 2026 retrospective study (n=47) showed symptom improvement with transdermal testosterone in breast cancer survivors, but called for placebo-controlled RCTs to establish safety 11

For Women Without Breast Cancer History:

• Testosterone therapy does not increase mammographic breast density 2
• Short-term transdermal testosterone does not appear to impact breast cancer risk 2
• Long-term breast cancer risk data are insufficient 2
• The only evidence-based indication is postmenopausal hypoactive sexual desire disorder after formal biopsychosocial assessment 2

Common Pitfalls to Avoid

1. Don't assume testosterone is "safe" based on observational studies in cancer-free populations—the biology in existing hormone-sensitive cancer is fundamentally different
2. Don't overlook the aromatization pathway—even if testosterone itself might have anti-proliferative effects, its conversion to estradiol remains a major concern 1, 5, 1
3. Don't use testosterone to treat endocrine therapy side effects in breast cancer patients—use alternative approaches like venlafaxine for hot flashes 1
4. Don't extrapolate transgender patient data to cisgender breast cancer patients—the risk-benefit calculus differs substantially 5

Bottom Line for Practice

The guideline consensus is clear and should be followed: testosterone should not be used in men with breast cancer and should be avoided in women with hormone-sensitive breast cancer. 1, 2, 1 While emerging research suggests potential protective effects in cancer-free populations, this does not translate to safety in patients with existing or prior hormone-sensitive breast cancer. The theoretical risk of promoting cancer recurrence through estrogen conversion outweighs potential benefits for symptom management, for which alternative therapies exist.