[farmerjohn1324]

I am making something that is a 7% w/w solution of lidocaine hydrochloride in water.

Can I safely store this in plastic, or does it need to be metal? If so, aluminum or steel?

[Arkcon]

This sort of topic is hard to know.  A quick search of MSDS doesn't give any specific information.  Furthermore, you didn't say which type of plastic.  And you haven't told us your application.  If I were preparing a standard, I'd keep it in glass.  If I were mixing it for sale, I'd be 110% sure it was compatible with packaging.  As it is, i just don't know.