Flagyl is a known causer of PN... In fact it can also create lesions in the central nervous system. But I don't think that happens so quickly.

http://en.wikipedia.org/wiki/Metronidazole

This drug increases aldehydes in the body... like alcohol does.

You might want to try high dose thiamine and see if that helps with this reaction. Benfotiamine which is an improved version, lasts longer in the body, and may work also.
300mg a day in divided doses for thiamine,
or 300mg a day of benfotiamine (may be taken all at once).

People vary in being able to metabolize aldehydes. This is a genetic thing. If you have had problems with alcohol causing flushing or hangovers with very small amounts, this may be your problem... a metabolic one. So this is worth a try.

Benfotiamine is not in stores, but you can find Doctor's Best brand on iherb.com or Amazon. Doctors Best is high quality and quite affordable. Many people with PN find it helps, not for everyone, but many. It is easily tolerated with little if any side effects. You can take with food too.