The interval of time when the drug is effective. is t ≥ 0
How to determine the interval of time when the drug is effective.
From the question, we have the following parameters that can be used in our computation:

First, we plot the graph of the function f(t)
See attachment for the graph
The negative values of t on the x-axis cannot be considered
This is so because time cannot be negative
Considering t ≥ 0, we have that
As the value of t increase, the function value increases
This means that the drug is effective for all non-negative values of t
Hence, the interval of time when the drug is effective. is t ≥ 0