It didn't have one before and the way it worked was that it would have its effect so long as there was enough to survive the first time chemicals were metabolized. .0001 units won't trigger this. 

Standard operating procedure in xenobio was when you got green/black slime mutation toxin you'd only get 1 unit, so you'd dilute that 1 unit in 30 or 40 units of water so that when you took a syringe and injected 5 units of that mixture in you there'd be enough to trigger the change and you could have multiple uses. If you over diluted the toxin it wouldn't work because it'd fall below that threshold. No one has ever treated this as an exploit.