Is there any risk of illness from consuming garlic?

Homemade garlic-in-oil (a mixture made with either whole or chopped garlic) can lead to serious health problems.  It is safe when made and used right away and if refrigerated and used within a week, but it can be dangerous if kept at room temperature or kept too long. Spores that produce the toxin that causes botulism cannot grow where there’s oxygen; when garlic is in oil, no oxygen can reach it. When buying commercial garlic-in-oil, check the list of ingredients. If the product contains salt or acid, then it’s been preserved and should be safe as long as you follow the storage directions. 

Garlic can interfere with certain prescription medications, including anti-coagulants and medicine used to treat HIV/AIDS. 

Garlic is a powerful herb. Raw, crushed garlic can irritate the skin.  Consuming raw garlic, especially in large amounts, could irritate the digestive tract.

Source(s):

Garlic Central  “Dangers”

http://www.garlic-central.com/dangers.html

Health Canada  Healthy Living  “Garlic-in-Oil”

www.hc-sc.gc.ca/hl-vs/iyh-vsv/food.../garlic-ail-eng.php