The certainty of evidence is donwgraded by study quality and by imprecision (small number of participants).
A Cochrane review «Treatments for breast engorgement during lactation»1 «Zakarija-Grkovic I, Stewart F. Treatments for breast engorgement during lactation. Cochrane Database Syst Rev 2020;(9):CD006946. »1 included 21 studies with a total of 2170 subjects. For breast pain and breast hardness, cold cabbage was a little more effective than routine care (n=152). For breast pain and breast hardness, herbal compress was more effective than hot compress or massage (n=500). For breast hardness, cold gel packs was slightly more effective than routine care (n=152).