Mastitis is inflammation of the breast, and it most commonly develops when milk isn’t fully drained during breastfeeding. Milk that stays trapped in the breast tissue creates pressure and swelling, which can progress to a painful infection. But breastfeeding isn’t the only cause. Mastitis can also affect people who aren’t nursing, particularly smokers.
How Milk Stasis Triggers Mastitis
The most common path to mastitis starts with milk that doesn’t move. When milk sits in the breast too long, whether from a missed feeding, a baby who isn’t latching deeply enough, or a duct that’s become blocked, the surrounding tissue swells and becomes inflamed. This is sometimes called “inflammatory mastitis,” and at this stage, bacteria may not even be involved yet. The breast hurts, turns red, and feels warm, but it’s the pressure and stagnation doing the damage.
If the milk continues to sit, the condition can worsen. Bacteria that normally live on the skin or in the baby’s mouth can enter through small cracks or fissures in the nipple and begin multiplying in the stagnant milk. At that point, inflammatory mastitis crosses into bacterial mastitis, bringing fever, chills, body aches, and fatigue that can feel like the flu. Left untreated, bacterial mastitis can progress further into an abscess, a pocket of pus that may need to be drained.
This is why stopping breastfeeding during an episode of mastitis tends to make things worse. Less drainage means more stasis, and more stasis means a higher risk of abscess formation. Continuing to nurse or pump from the affected breast, even though it’s painful, is one of the most important steps in recovery.
Risk Factors During Breastfeeding
Anything that prevents milk from flowing freely raises your risk. The most common culprits include:
- Poor latch: When a baby doesn’t latch correctly, certain areas of the breast don’t get fully drained during feedings.
- Cracked or sore nipples: Damaged skin creates an opening for bacteria to enter breast tissue. Yeast infections on the nipple can cause similar cracks and also contribute to milk stasis.
- Skipped or shortened feedings: Going longer than usual between feedings, or cutting sessions short, lets milk accumulate.
- Previous mastitis: If you’ve had it before, you’re more likely to get it again.
- Fatigue and stress: Being overtired or poorly nourished weakens your body’s ability to manage early inflammation before it escalates.
- Smoking: Tobacco use is a risk factor for both lactational and non-lactational mastitis.
One common belief is that tight bras or underwire can compress milk ducts and cause blockages. The Mayo Clinic lists pressure on the breast from a tight bra or seatbelt as a risk factor. However, other medical sources push back on this, noting there is no scientific evidence that tight-fitting bras cause plugging or mastitis. The internal structure of the breast isn’t easily compressed by external clothing in a way that blocks ducts, much like a tight belt can’t cause an intestinal blockage. That said, if a bra is digging into breast tissue that’s already swollen or engorged, easing the pressure certainly won’t hurt.
How It Feels
Mastitis typically affects one breast at a time. The earliest sign is usually a localized area of tenderness, warmth, and redness. You might notice a hard, wedge-shaped area that feels swollen. In its milder inflammatory stage, this may be the extent of it.
When infection sets in, the symptoms become systemic. Fever (often above 101°F), chills, muscle aches, and deep fatigue are common. Many people describe it as feeling like they suddenly came down with the flu, except one breast is visibly red and painful. Symptoms tend to come on quickly, sometimes within hours.
Because mastitis exists on a spectrum, from simple ductal swelling to full bacterial infection, mild cases can sometimes resolve with one to two days of conservative care: frequent nursing or pumping, rest, gentle massage toward the nipple, and warm compresses before feeding. Antibiotics aren’t always necessary for the inflammatory stage. But if fever and worsening symptoms persist beyond a day or two, the infection likely needs treatment.
Mastitis Without Breastfeeding
You don’t have to be breastfeeding to get mastitis. Non-lactational mastitis most often takes the form of periductal mastitis, where the milk ducts just behind the nipple become inflamed and sometimes infected. Smoking is the dominant risk factor. In one study of 139 patients diagnosed with periductal mastitis, 89 percent were smokers, compared with 39 percent of age-matched controls.
The connection appears to be chemical. Toxic substances in cigarette smoke concentrate in the breast, particularly in the ducts near the nipple. A nicotine byproduct called cotinine reaches higher levels in those ducts than in the bloodstream. Researchers believe these substances directly damage the duct lining, causing tissue breakdown that invites infection. Non-lactational mastitis can recur and is sometimes harder to treat than the breastfeeding-related kind, partly because the underlying duct damage from smoking tends to persist.
Reducing Your Risk
For breastfeeding parents, the most effective prevention is making sure milk drains well and often. That means feeding on demand rather than on a rigid schedule, making sure your baby’s latch is deep and effective (a lactation consultant can help troubleshoot this), and avoiding sudden long gaps between feedings when possible. If you’re returning to work or changing your routine, gradually adjusting your pumping schedule helps your body adapt without dangerous engorgement.
Probiotics have drawn research interest as a preventive measure. A meta-analysis of six randomized controlled trials found that taking oral probiotics during pregnancy cut the incidence of breastfeeding-related mastitis roughly in half. The probiotics also appeared to reduce bacterial counts in breast milk in both healthy individuals and those already experiencing mastitis. However, the evidence base is still relatively small, and one large Norwegian cohort study of over 50,000 women actually found a positive correlation between probiotic milk intake and mastitis, though the study authors noted this likely wasn’t a causal relationship. Probiotics show promise but aren’t yet a proven standard recommendation.
For non-lactational mastitis, quitting smoking is the single most impactful preventive step. As long as the duct-damaging chemicals keep concentrating in breast tissue, the cycle of inflammation and infection is likely to continue.