Naturally injected dopamine is has pretty dangerous nature - the current level of "pleasure" is memorised as base(means nullified) and user has to inject more and more even to maintain static level of "enjoyment". Thus junkie is dragged into consuming more.
And, because this injection is not strictly regulated inside, but injected uncontrolled from outside - only by person will and mind, the injected amount is not correlated to anything, does not follow the rule "there is no free dinner" - meaning disbalance of fluids. The effect is similar to overeating.
Additionally, as with everything in this life, "if its not used, it is not needed". Try not to walk for a week. You legs will atrophy. The effect on internal organs, is that every subsystem that is not used, will atrophy. If person injects external drug and this drug replaces existing connections, every unneded connections upstream, inside the body, will be not used anymore and will atrophy. Watchout how muscular, testosterone consuming bodybuilders get small "eggs"; and how they start to *SING* like girls(thanks to atrophied natural production of testosterone due to external injection; and thanks to still produces female hormone osterogen), if they break their external *addiction* chain.
Combined effect means, the junky will become addicted, sooner or later, his older "independent" internal injection systems will atrophy or vanish, he will be forced to consume more and eventually drag all his life.
Personally, when I breathe air, it happens alone, Im not charged, it is naturally necessary and replaces no existing systems. I like this drug.
Watch those who replace their internal natural endorphine release by external injection of endorphine release triggering coffein, drinking coffee at morning - that is. Soon, they are unable to wake up without a cup of coffee.