Very interesting concept, but feels terribly underplayed.

You've got an interesting premise: A girl is introduced to a drug that can make her more curvaceous, and causes her to take elephantine level dumps; yet it feels rushed. Instead, you should have the first chapter be like the first week or so, then skip around a little, while having a few chapters describe how Jenny's body adapts and changes (maybe even have instances where her butt and breasts expand on her pleasurably).

The dump should also be given more detail and graphicness. Like her panties get filled to the point they explode on the first filling. Also, since the building is "state of the art", it should be able to accomodate for the massive shit she's giving birth to. You also need to give more detail as to what the company is after with these experiments, and maybe have Jenny meet/see other test subjects (like a girl who gains two feet in height and can shit a turd roughly the thickness of an average human arm with little effort). For an idea, maybe the ultimate side-effect is gradually giantism, causing Jenny to become a giantess who can't fit into normal places any more.

Finally, you need to fix some spelling errors, as some spots were missing words, or had the wrong word and thus caused the paragraph to have an sort of bad english feel to it.