This is a single-center, double-blind, randomized clinical trial to explore the efficacy and safety of external mastalgia-oil versus placebo in premenopausal women with severe mastalgia.

Inclusion Criteria:

• Premenopausal females aged ≥ 18 years old
• Pre-existing regular menstrual cycle (28 ± 5 days) with mastalgia lasting for at least 6 consecutive menstrual cycles; with severe mastalgia [visual analog scale (VAS) score ≥ 4 for at least 7 days] and without chest wall pain in the baseline menstrual cycle (nonmedicated menstrual cycle)
• With breast ultrasound: BI-RADS classification 1-3; if mammography is required: also BI-RADS classification 1-3
• Willingness to follow up and complete required tests; ability to complete questionnaires independently or with assistance
• Willingness to use effective contraception (vaginally administered estrogen and hormone-coated IUDs are permitted) prior to study entry, during study participation, and for three months after discontinuation of the drug; negative pregnancy test in females of childbearing potential
• Requisite laboratory values:white blood cell count: ≥ 4.0 × 10^9/L, absolute neutrophil count: ≥ 2 × 10^9/L, platelet count: ≥ 100 × 10^9/L; hemoglobin: ≥ 110 g/L; aspartate aminotransferase and alanine aminotransferase: ≤ upper limit of normal, alkaline phosphatase: ≤ upper limit of normal, blood total bilirubin: ≤ upper limit of normal; serum creatinine and blood urea nitrogen: ≤ upper limit of normal

Exclusion Criteria:

• With suspected breast disease (e.g., mastitis, malignant breast tumor) other than breast pain or benign breast cysts, or with other malignant tumors
• Use of hormonal medications (e.g., oral contraceptives, glucocorticoids, etc.), sex hormone modulators (SERMS, AIs, SERDs, etc.), B vitamins, essential oils, nonsteroidal anti-inflammatory drugs, or higher pain relievers in the past 3 months; use of tamoxifen or analogs in the past 6 months
• Prior history of breast surgery (including minimally invasive surgery), history of malignancy, or history of thromboembolism within the last 5 years (history of varicose veins and superficial phlebitis allowed)
• Smokers; essential oils or placebo systemic/skin sensitization
• With serious primary diseases of the heart, liver, kidneys and hematopoietic system, or mental illness, etc.
• Accompanied by skin damage on the breast that destroys the stratum corneum (e.g., eczema, ulcers)
• Serious or uncontrolled infections that may interfere with study treatment or assessment of study results, including but not limited to active hepatitis viral infection, human immunodeficiency virus antibody positivity, syphilis spirochete antibody positivity with evidence of active infection, and pulmonary infections
• Subjects who are breastfeeding or pregnant at the time of screening (of childbearing age and with serum human chorionic gonadotropin test results higher than the reference value); subjects of childbearing age who have had unprotected sex with a heterosexual partner within 2 weeks prior to the screening; subjects of childbearing age who are planning to prepare for conception, become pregnant, breastfeed, or donate their eggs during the test period or within 3 months of the end of the test; and those with implanted breasts
• Decrease in total breast pain score by more than 25% during the placebo lead-in cycle. These patients will not receive subsequent randomization and treatment.