Heating garlic can reduce its beneficial effects.

As little as 60 seconds of microwave heating or 45 minutes of oven heating inactivated the allinase (garlic’s principal active ingredient), thereby eliminating garlic’s ability to protect rat mammary epithelial cell DNA in vivo against breast cancer. Letting the crushed garlic stand for ten minutes before microwaving for 60 seconds prevented the total loss of anti-carcinogenic activity.