Allicin is a component of garlic. It’s been well studied for its antimicrobial properties, and works to reduce microbes from the Pseudomonas, Streptococcus, and Staphylococcus families, among others [Source: PubMed].
It also works to reduce Candida albicans (a yeast that can cause thrush in women and also lives in the gut), and has been shown to kill parasites like Entamoeba histolytica and Giardia lamblia [Source: PubMed].
‘SIBO Queen’ Allison Siebecker says on her website that she uses a combination of allicin and neem to treat methane-dominant SIBO, with good results