Time-varing covariate and renal function as a covariate

Dear All, I have enjoyed the recent discussions on time-varying covariates and renal function as a covariate. The two topics emerged from a single thread. Not to belabor this, but I thought it interesting to bring these back together in order to pose a question. How would one deal with renal function and "size" as time varying covariates if administration of the drug in question results in changes in weight over time? Kind regards, Matt