Garlic has been recognized as a favorable natural medicine against a large number of pathologic conditions. The major component of garlic is allicin, which could effectively inhibit bacteria and viruses. By this anti-microbial activity, allicin may be an effective therapeutic candidate to promote ulcer healing.
Fujisawa and co-workers at the Nihon University Graduate School of Applied Life Sciences, Japan, demonstrated that allicin had only weak antibacterial activity . They compared the antibacterial activities of freshly prepared garlic extract and clinically useful antibiotics. The phytochemicals had 1-2% of the potency of streptomycin against S. aureus, 8% of that of vancomycin against S. aureus and only 0.2% of that of colistin against E. coli. The antibacterial activity of allicin was completely abolished by cysteine and other SH-compounds, indicating that the oxygen in the structure -S(=O)-S- of allicin liberates the S-allyl moiety, which might act against bacteria.
A study conducted at the University of East London, UK, concluded that an aqueous allicin solution and a novel gel formulation with allicin was bactericidal against of Lancefield group B streptococci isolates . The team of researchers, led by Dr Cultler, investigated the in vitro activity of a novel allicin extract in aqueous and gel formulation against 76 clinical isolates of Lancefield group B streptococci.
Bakri and Douglas at the University of Sheffield, UK, found that garlic extract inhibited the growth of oral pathogens and certain proteases and may have therapeutic value, for example for the treatment of periodontitis . They found that an aqueous garlic extract of garlic, containing 220 microg/ml allicin, inhibited the growth and killed most of the organisms tested.
Garlic is alleged to have antiviral properties that relieve common cold. However, Lissiman and coworkers at the University of Western Australia could not find clinical evidence regarding the effects of garlic in preventing or treating the common cold . They reviewed five studies about the effectiveness of garlic for either the prevention or treatment of the common cold. But only one study met the inclusion criteria. This study randomly assigned 146 volunteers to a daily garlic supplement, containing 180 mg of allicin, or a placebo for 12 weeks. This study suggested that garlic may prevent occurrences of the common cold, but more studies are needed to validate this finding.
 Antibacterial potential of garlic-derived allicin and its cancellation by sulfhydryl compounds. Biosci Biotechnol Biochem. 2009 Sep;73(9):1948-55.
 In vitro activity of an aqueous allicin extract and a novel allicin topical gel formulation against Lancefield group B streptococci. J Antimicrob Chemother. 2009 Jan;63(1):151-4.
 Inhibitory effect of garlic extract on oral bacteria. Arch Oral Biol. 2005 Jul;50(7):645-51.
 Garlic for the common cold. Cochrane Database Syst Rev. 2009 Jul 8;(3):CD006206.