I took Step 1 a year ago, but unless anything's changed, a good chunk of it is unnecessary. You don't need to know the mechanism of action of most of the drugs (maybe be generally familiar with ethosuxamide and the big 3: Valproate, carbamazepine, and phenytoin). Side effects are important, but again, broadly:
-Phenytoin: Fetal hydantoin syndrome, folate deficiency
-Carbamazepine: Neutropenia
-Valproate: Pancreatitis, hirsutism
-Others in general: Steven-Johnson Syndrome
Indications are also important. Besides the big 3 again, must-know indications are ethosuxamide for absence seizures and MgSO4 for eclamptic seizures.
All in all, you'll see what's ultimately necessary when you do UWorld and NBME's.