Dan 5658 is 10 mg cyclobenzaprine (generic Flexeril).

It's a muscle relaxant used for short-term treatment of muscle spasms and other painful muscle injuries.

http://www.pharmer.org/node/1487

This is pretty obvious, it must contain 5658 guys named Dan, all compacted into pill form so you can fulfil your daily requirment for guys called Dan, and not develope a nasty little condition known as 'Post-Danial-Withdrawl' in which you get all weak and spindly, turn yellow, and grow little feathers on top of your head like a duck, due to a defficiancy of Dans in your diet.

(Sorry, I could'nt resist.)