dan 5658 is cyclobenzaprine (flexiril) which is a muscle relaxer.
It can be used for muscle spasm or to aide with sleeping problems in people with chronic back pain.
Just 10mg can knock you out for 12 hours. Imagine not sleeping for two whole days and then finally going to bed, they make you feel like that within a couple of hours. You become unable to stay awake (be cautious if you are going to drive or operate machinery, or even just cut some vegetables).
I stopped taking them because they made me feel very groggy and dry the whole next day.
If someone is taking them without a prescription (like for recreation) then they need to stop because of serious side effects that can occur at even just two doses (20mg). Irregular heartbeat or even heart failure and difficulty breathing or even passing out and failing to breath can occur.
DO NOT MIX WITH ALCOHOL. Not fun, not cool, just don't do it.