Our Gemara quotes a Mishna from Massekhet Nega’im, which teaches that there are three people who shave their heads and fulfill a mitzva by doing so. They are:
- A nazir
- A metzora – a leper
- The levi’im, for when the Levites were first consecrated for work in the mishkan their bodies were shaved (See Bamidbar 8:7).
In all of these cases, the obligation is to shave the head totally with a razor. If one leaves even two hairs, or removes the hair by other means, he does not fulfill his obligation.
In response to the challenge that these laws are obvious, the Gemara suggests that we may have thought that the obligation was simply to remove the hair by any means, and there was no specific obligation to do so by mean of a razor, so that even nasha – depilatory cream – could be used. The Mishna therefore needed to teach us this rule.
The source of the term “nasha” is unclear, and some prefer a variant reading that appears in the Mishna in Nega’im that the word is nasham, which is understood to mean that it is medicinal cream of some sort.
Even today we find that creams are used for hair removal, which work on a basis of chemicals that break down the hair so that it can be removed simply by brushing it off. Generally speaking sulfur salts are the active chemical in such creams, and it is possible that nasha was made from similar compounds, perhaps with the addition of lime. Nasha successfully removed hair for an extended period of time, and sometimes the compounds were so strong that they permanently removed the hair.
In truth, the nazir is not obligated to remove his hair, although, as noted, he fulfills a mitzva by doing so, and should do it even if a significant amount of time had passed since his nezirut was completed.