The American Society for the Prevention of Cruelty to Animals (ASPCA) has a discussion about garlic on their website which suggests that it may potentially be toxic with chronic exposure in any form. 

Go to the following website for more information:

 http://www.aspca.org/site/PageServer?pagename=pro_apcc_ask_peoplefood&s_state=garlic
 
The site states:

 “The bottom line is that we do not definitively know at what dose any given dog may experience problems. An occasional low dose, such as those found in most commercial pet foods or treats, would not likely cause problems.  A conservative approach might be to avoid exposure to more concentrated garlic-based products.”