This systematic review of 5 randomized placebo-controlled trials qualitatively assessed the effectiveness of prebiotics and probiotics for reducing the symptoms of lactose intolerance. Two studies assessed a prebiotic, and 3 assessed probiotics. A brief description of each study is given as follows:

Savaiano, et al., 2015: 85 participants received RP-G28 (a prebiotic) or a placebo for 35 days.^[1]
Chey et al., 2020: 377 participants received RP-G28 or a placebo for 30 days.^[2]
Ojetti et al., 2010: 60 participants were randomized to receive tilactase (comparable to a lactase enzyme) 15 minutes before a lactose breath test, Lactobacillus reuteri DSM 17938 (a probiotic) for 10 days, or a placebo for 10 days before a hydrogen breath test.^[3]
Aguilera et al., 2021: 50 participants performed hydrogen breath tests immediately after ingestion of ice cream containing 20 grams of lactose with a lactase tablet, a low (10⁵ CFU per gram) or high (10⁷ CFU per gram) dose of Bifidobacterium bifidum 900791, or ice cream without lactase or a probiotic. Subsequently, 45 participants were randomized to consume the ice cream daily with the low-dose probiotic or a placebo for a month.^[4]
Pakdaman et al, 2016: 38 participants took either Lactobacillus acidophilus DDS-1 or a placebo for 4 weeks. ^[5]

Are prebiotics and probiotics effective for lactose intolerance symptoms? Original paper