You can assign your Sim the Clubber Trait in CAS.
This trait alters need decays, and gives your Sim whims related to clubbing, drinking and doing drugs. A Sim with the clubber trait will not react negatively to witnessing Drug Deals, or watching people doing drugs, and is easier to assign as a drug customer.
Straight Edge Trait
This is basically an anti-druggie trait.
You can assign your Sim the Straight Edge trait in CAS. This trait will conflict with the NPC Drug Dealer trait (obviously). A Sim with the Straight Edge trait cannot do any drugs, buy any drugs, sell any drugs, share any drugs, make any drugs or drink bar drinks, etc.
They will also get quite infuriated if a Sim tries to sell them drugs, and this will result in a massive relationship loss between the Straight Edger and the Dealer. If there are other Sims in the proximity who are Straight Edge, both of the Straight Edgers will receive a happy buff as they like to be in each others company.
Go to Adderall to read about the ADHD trait.