A small study that found an inverse link between garlic and carcinogens with the use of a novel testing method for both markers in urine was published in Analytical Biochemistry, an international journal devoted to biology and biochemistry.
Lead researcher, Earl Harrison, PhD at Ohio State University's (OSU) Comprehensive Cancer Center explained, "what we were after was developing a method where we could measure in urine two different compounds, one related to the risk for cancer, and the other, which indicates the extent of consumption of garlic."
Harrison continued, "the precise mechanism by which garlic and other compounds affect nitrosation is under extensive investigation, but is not clear at this time. What this research does suggest, however, is that garlic may play some role in inhibiting formation of these nitrogen-based toxic substances. This was a very small pilot study, so it's also possible that the more garlic you have, the better it would be."
The good news is, for now, "... if you like garlic and you like garlic-containing foods, go out and have as much as you want. There's no indication it's going to hurt you, and it may well help you."
Full study: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6W9V-4WW2SN5-1&_user=10&_coverDate=11%2F15%2F2009&_rdoc=15&_fmt=high&_orig=browse&_srch=doc-info%28%23toc%236692%232009%23996059997%231492118%23FLA%23display%23Volume%29&_cdi=6692&_sort=d&_docanchor=&_ct=23&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=81c77336b30de4e263ecc5ac7edd89ca