Could a chemical compound change a human body in a way that a regular quantity of that compound is needed to stay alive after the first dose?
The short answer would be yes, there exist already such chemicals (and are more common than I thought, in example alchol), however those chemicals requires an extended period of time in wich are abused.
Similiary there are substances that replaces human biologic mechanisms and though after a "long training" time those substances may cause the body to stop production of important substances (alchol is one, but also insuline, benzos, opiates).
To get inspiration I also asked a similar question on Health SE
However all those substances takes a long period of time before having effect, so is there any real drug/medicine from wich I can take inspiration that needs a long detox period (with medical assistance) even after the assumption of a relatively small dose?
Such drug/medicine would be required in example to control special agents, instead of poison them and givin them the antidote, a secret agency may simply just give a known substance once, after that they need that substance to avoid death.
The fictional drug should be based on some mechanism already present in real medicines so that it have a real plausible explaination.
The afore mentioned substances may have lethal withdrawal, however a medical assisted detox period is enough to avoid death, in my case I want avoiding death is not trivial and the most easy way is just keeping on regular doses.