DISEASE CHARACTERISTICS:

* Histologically confirmed primary invasive adenocarcinoma of the breast

* Stage I, II, or IIIA disease
* No metastatic disease
* Must have undergone modified radical mastectomy or breast-sparing surgery and recovered
* Estrogen-receptor positive (ER+) and/or progesterone-receptor positive (PR+)
* Currently taking a third-generation aromatase inhibitor (AI) \[e.g., anastrozole (Arimidex®), letrozole (Femara®), or exemestane (Aromasin®)\] for ≥ 90 days prior to registration with plans to continue for ≥ 180 days after registration
* Must have completed the S092 Brief Pain Inventory (BPI)-Short Form within the past 14 days, and must have a worst pain/stiffness of ≥ 5 on the BPI (item #2) that has started or increased with AI therapy

PATIENT CHARACTERISTICS:

* Postmenopausal
* Zubrod performance status 0-2
* Willing to submit blood for serum-free estradiol, total estradiol, serum inflammatory markers (IL6, TNF-α, CRP), DHA and EPA, lipid profile (LDL, HDL, triglycerides), and DNA analysis (CYP19A1)
* Able to complete study questionnaires in English
* At least 5 years since other malignancy except adequately treated basal cell or squamous cell skin cancer, in situ cervical cancer, ductal carcinoma in situ of the breast or adequately treated stage I or II cancer from which the patient is currently in complete remission
* Patients must not have a known allergy to soy, given that the placebo is suspended in soybean oil

PRIOR CONCURRENT THERAPY:

* See Disease Characteristics
* At least 3 months since prior omega-3 fatty acid supplements and must agree to refrain from omega-3-fatty acid supplements from sources outside of this study
* More than 28 days since prior investigational agents
* No other medical therapy, alternative therapy, or physical therapy for joint pain/stiffness within the past 30 days
* Patients must not be on anticoagulation medication (i.e., heparin/warfarin) because of increased risk of bleeding within 28 days prior to registration
* Patients must not have a history of bone fracture or surgery of the afflicted knees and/or hands within 6 months prior to registration
* Patients must not be on narcotics within 14 days of registration
* Patients may have received corticosteroid treatment; however, the following criteria apply:

* Patients must not have received oral or intramuscular corticosteroids within the 28 days prior to registration
* Patients must not have received intra-articular steroids to the study, or any other, joint within 28 days prior to registration
* Patients must not have received topical analgesics (e.g., capsaicin preparations) to the study joint or any other analgesics (e.g., opiates, tramadol; with the exception of nonsteroidal antiinflammatory drugs (NSAIDs) and acetaminophen) within 14 days prior to registration