Tapering off the smack can be done, but you would have a much higher sucess rate if you used the meth
If you cannot find meth/sub , what your going to want to do is slowly start lowering the dose you are taking.
So, if swim
.5g a day, Monday and Tuesday she should only snort .4 , Weds and Thurs do .3 and so on, continue to lower your dose every two days (or whatever works for you) untill you completely stop.
Now, doing this will not eliminate all withdrawl symptoms but if done right it should make them managable.
Wish you the best of luck getting off the smack.