Experiencing soreness localized to only one breast is a common physical concern. This symptom, known as unilateral mastalgia, is defined as pain or tenderness concentrated in a single breast area. While any new symptom can cause anxiety, unilateral breast soreness is rarely associated with a serious underlying medical condition and is often explained by benign causes. This article explores the reasons why discomfort might be isolated to one side.
Temporary and Non-Medical Causes of Unilateral Soreness
One frequent explanation for one-sided tenderness relates to normal hormonal fluctuations. Cyclic breast pain (mastalgia) may be felt more intensely in one breast if that side’s tissue is more sensitive to the monthly rise in estrogen and progesterone. This discomfort often begins around ovulation and subsides with the start of menstruation, indicating a sensitivity difference in the tissue itself.
Another common cause is pain originating outside the breast tissue, referred to as extramammary pain. The pectoralis muscles lie directly beneath the breast and can become strained from activities like weightlifting, strenuous coughing, or sleeping in an awkward position. This muscle soreness or inflammation of the rib cartilage, called costochondritis, is typically unilateral and can be mistaken for breast pain due to the close proximity.
The fit of a bra can also cause localized soreness. A poorly fitted bra, especially one that is too tight or has an underwire that digs into the chest, can compress tissue or irritate the underlying ribs. Conversely, insufficient support from a loose bra can overstretch the Cooper’s ligaments that support the breast, resulting in unilateral aching or soreness. Minor physical trauma, such as a bump from a seatbelt or an impact during sports, can also cause transient, localized pain and bruising that quickly resolves.
Inflammation and Infectious Causes
When pain is accompanied by warmth, redness, and swelling, an inflammatory or infectious process is likely. Mastitis is an inflammation of the breast tissue that most commonly affects those who are lactating (lactational mastitis). This infection occurs when bacteria enter the breast, often through a cracked nipple, leading to a painful, wedge-shaped area of redness, along with systemic symptoms like fever and body aches.
A clogged milk duct frequently precedes lactational mastitis, causing a tender, firm lump without immediate fever. Non-lactational mastitis can also occur in individuals who are not breastfeeding, sometimes linked to duct ectasia or periductal mastitis. This chronic inflammation often affects the ducts near the nipple and is more common in postmenopausal women or those who smoke, manifesting as a painful, firm lump near the areola.
Breast cellulitis is a bacterial infection of the skin and underlying soft tissue. This condition is often seen after a break in the skin from surgery, injury, or piercing, and presents with rapidly spreading redness, warmth, and intense tenderness. Cellulitis may also involve red streaking on the skin and requires prompt medical treatment with antibiotics to prevent abscess formation.
Pain Caused by Structural Changes
Unilateral soreness can signal a change in the physical structure of the breast tissue, often involving a mass. Simple cysts, which are fluid-filled sacs, are a frequent cause of pain, particularly in women aged 35 to 50. A cyst becomes painful when it develops rapidly and causes pressure on the surrounding tissue, resulting in a tender spot that may fluctuate in size with the menstrual cycle.
Fibrocystic changes describe hormonal changes that result in lumpy, dense breast tissue. While this condition typically affects both breasts, the nodularity and associated tenderness are often more pronounced and painful on one side. This discomfort is generally diffuse across the affected area and worsens in the days leading up to a period.
A fibroadenoma is a common, solid, non-cancerous mass composed of glandular and connective tissue. Although typically painless, they can cause discomfort if they grow large enough to compress nearby nerves or breast tissue. These masses are often described as firm, rubbery, and highly mobile, sometimes termed a “breast mouse.” Mammary duct ectasia involves the widening and thickening of a milk duct, often presenting as a lump near the nipple or areola, accompanied by spontaneous discharge and pain in the single affected duct.
When to Seek Professional Medical Guidance
While most instances of unilateral breast soreness are benign, certain symptoms warrant evaluation by a healthcare provider. Any new, persistent lump that does not fluctuate with the menstrual cycle should be examined, especially if it feels hard or fixed in place. Pain that lasts for more than one to two full menstrual cycles or that persists after menopause also requires investigation.
Specific changes to the skin or nipple require evaluation. These include skin dimpling, puckering, or a texture resembling an orange peel (peau d’orange). Nipple discharge also needs evaluation, particularly if it is clear or bloody, occurs spontaneously without squeezing, and comes from only one duct or one breast. Signs of a worsening infection, such as high fever, chills, or rapidly spreading redness that does not improve after a day or two of home care, should also be addressed by a doctor.