After Jerry picks up his daughter from Sandra to babysit her (at approximately [1:01:40]), he can be seen driving down the road sitting on the right hand side of the car. This is not consistent with other scenes in the movie and furthermore, the driver is clearly not Ben Stiller.
When Jerry is at the methadone clinic, the doctor is checking boxes on a form he's filling out. The questions he asks Jerry do not match the boxes he checks.
Sandra could have just taken Jerry's car keys, and have him stay at her home to watch Nina, this way he has no means to go out and buy drugs.