Modifying initial set of Initial values inODE15s

My thesis involves soling stiff ODEs and I am using ODE15s. At some particular time 't' I want to add some value to one of the variables used. Basically I want to see the dosing schedule for therapeutic applications.Kindly guide me what to do If someone has the code similar to that or knows how to modify the ODE15s solver please comment.

Thank You

[Torsten]

Use the "Events" Option of ODE15S:

http://de.mathworks.com/help/matlab/math/ordinary-differential-equations.html#f1-669698

Best wishes

Torsten.