This feature would be VERY useful especially when you like to taper or substitute a drug.
In that cases, you have planned changes of the amount or the kind of a medication, i.e. you know at a special day you have to lower (or to rise, when the dose must be increased) the dose of a medication, for example for one week 30mg, next week 20mg, the following week 10mg, then a substitution by another drug.
To be able to map this, a planned start for a medication would be necessary, which unfortunately currently isn't possible.
Anyway, RXDroid is a great app - I really like it!