Secret to TS prior drug use not disclosed

I have held my current Secret level clearance for 3 years. However, while updating my eqip for a pending TS clearance I realized that I did not disclose prior marijuana use. The use occured 1.5 years before my Secret eqip was filed. My employer send me an SF86C to update my clearance with instructions to only update the address and employment sections. As a result I completely failed to review the other sections and appropriately update the drug use section.

My question is now that I know. When I update the information what is likely to occur? Will my current clearance be revoked for a lack of remembering?

Is there a reason why you failed to put in your drug use when you originally filled out your SF86?