Is the doctor you're referring to a Primary Doctor or GP? For most Primary doctors, it's often just a guessing game. And Lexapro is one of the most common anti-depressants.
To start on a 5mg dosage is awfully small - almost insignificant - according to an oncology nurse I know. My talk psychologist, after several sessions with him, suggested a 20mg dosage - and that was for the first prescription.
I have been on the 20mg dosage of the generic lexapro for close to 6-months. It has helped my emotional state and with no noticeable side effects. But then, I've been on cancer meds for over 3-years - and those SE's are far greater than Lexapro!
It has been an effective and low-cost medication for me. It gradually helped me over the first 30-days or so - and continues to help now!
Hope this is of some help.
Rob & Gizmo