how to account for duration of drug exposure

[shivaa]

I am studying impact of a medication on survival outcomes. each patient was on this particular medication for different duration of time and outcomes depend on duration of exposure. so i think analysing by users vs non-users will not be adequate. How do I account for duration of exposure into the survival analysis - cox proportional hazard model?

[Reeza]

What about including it as a variable? But it may also be time dependent so consider checking the time dependency aspect. 

[shivaa]

thanks for the reply. 

Could I use the 'usage of drug' as a time dependent co-variate in the model and will that account for duration of exposure. I know it solves the problem of immortal time bias..