Do you mean mephadrone verses methadone.
If so the first is an analogue of amphetamine, more like mdma than sulphate, with a distinct smell and white crystilne structure.
The second is likely to be a green liquid, a crap synthetic derivitive of opiates.
Both can be dangerous, especially as our (British) pathetic drug laws literally force people to either:
queue to use filthy club toilets, in a state of paranoid expectation, to snort unknown quantities of unknown chemicles. (In a sensible society we would be allowed to know the chemicle is what we wanted and measure a safe dose without worrying about the club security or external law. Sorry about the rant.)
or use the crap (once again due to our pathetic drug legislation) that doctors prescribe to mono-drug users to turn them into poly-drug users, done under the illusion that this constitutes 'treatment'.
This too creates dangers because a lot, if not most methadone is sold on to others (who may have no tollarence to opiates) so the drug user can buy their drug of choice.
Some lucky people do get real treatment choices, (when they ask for help for their opiate addiction) which prevents the creation of the so called 'grey' market, but they are the exception rather than the rule, unfortunately.
Two vastly diffent compounds both made more dangerous by existing drug legislation.
