Complex sentence: Within a very short time of regularly using, after the user would shoot up, which would physically invigorate their bodies for an undetermined amount of time, as the drug wore off, they would begin to wither to an emaciated state within hours.
Split into simpler sentences: Within a very short time of regularly using, the user would shoot up, which would physically invigorate their bodies for an undetermined amount of time. As the drug wore off, they would begin to wither to an emaciated state within hours.