usually heroin is not just around for a minute and it will either be professionally done, or almost as good, and people dont usually put a lot in anyway because of cost and overdose reasons.
if you are snorting it you probably dont have much to worry about because you dont get as much of an effect and you have a wider margin ef error, i would say take a 1/5th of the total and do that, if its not enough do another 1/5th, and you need to wait at least 5-10minutes between these so you dont do more before the last takes effect.
Keep this in mind, its all set out for the shooters, so you have less than you think there. to someone with no tollerance a "bag" could very well kill you especially shot up. usually you get 2 "hits" from a bag, and this varies so much you cant tell whats what maybe you get 5-7 good hits, its cut to hell so you dont have to worry as much but remember to do at most a 1/5th and you shouldnt be far off.
If you want to shoot it your taking a chance if you dont know the purity, i would snort a particular variety first, and then do a bit less than what you got optimal effects this way to load up the rig.
Remember shooting is quite potent and basically the only way to get your moneys worth. You might try a 1/10th if its really good at 1/5th snorted, or if you needed the whole bag snorted maybe you might try a 1/4th injected, it will be a lot less.
never shoot half a bag right off ever unless you are *DAMN* sure its the same, this is the danger of it being illicit, heroin is no more dangerous than any opiate, but no other opiate is distributed the same and so variable, so this is the danger, not the chemical itself
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~The Recreational Pharmacist
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