“Chai” chains, “Star of David” chains, and even “Hamsa” vampire-protection chains. We are all familiar with the various physical symbols people like to use to express their Jewish identity and pride. Just google “mezuzah chain” and you will see that a mezuzah pendant seems to be a popular member of the chain gang as well. Because of its ascribed powers of protection, some people have also been known to keep a mezuzah in their car as well.
But how “kosher” is it to use a mezuzah as an amulet or lucky charm? And even if one’s respectful intention is purely to proclaim his faith and Jewish identity, is it disrespectful to hang around his neck a holy scroll containing Torah verses and multiple Names of G-d?
One thing is for certain: The Torah clearly states that the mezuzah’s place is on the doorpost. Thus, one who wears it, carries it, or keeps it in his car fulfills no mitzvah. Nevertheless, some authorities accept that the mezuzah’s protective power may still be in effect even when it is not in active service. Indeed, some sources suggest that in ancient times some people would insert a mezuzah into a hollowed-out chamber in their walking sticks for protection. Similarly, the Talmud records that tefillin retain protective power even when not strapped in place.
Practically speaking, the authorities debate whether hanging a mezuzah on the neck is considered an act of disrespect. Some cite the Talmud’s prohibition against hanging tefillin from a peg and maintain that this concern would apply to hanging a mezuzah around the neck. Certainly, this would apply to hanging a mezuzah in a car (next to the dice?). But others assert that, on the contrary, hanging a mezuzah on the body is a sign of endearment. Indeed, the Talmud records that a Jewish king would hang a small Torah scroll from his arm in fulfillment of the Divine command that he should carry the Torah with him constantly.
Yet even those who allow this practice insist that the scroll must be encased in two coverings if it is to be worn while entering a bathroom. Some require that the mezuzah pendant be taken off in this situations. Additionally, it should not be present in a bedroom during marital relations unless it is encased in two coverings.
It must be noted that some authorities have expressed strong hesitations about wearing mezuzot as necklaces, even when these technical considerations are met. They fear that people will mistakenly feel that, since they are wearing a mezuzah, they no longer need to affix mezuzot on their doorways. They also suspect that some people will intend that words of Torah written in the scroll should heal them from sickness. Words of Torah may have protective powers but using them directly as “medical” instruments is a degradation of the honor due them, except in cases of dire illness.
Because of these and other concerns, wearing a mezuzah necklace is not a common observance in our times. It may be part of the “chain gang”, but it is not a link in the chain of tradition.
 Mishnah, Keilim 17:16
 Berachot 23b
 Sanhedrin 22a; Igros Moshe Y.D. 2:141
 Mishnah Berurah 43:25
 Shevet Halevi 2:149.
 Shulchan Aruch Y.D. 179:11-12.