Go To Rehab
If your Sim is addicted to a drug he or she can decide to go to rehab. You do this by clicking on your addicted Sim (while active), go to the “Drugs” pie menu, “Rehab” sub menu and select “Go to Rehab”.
The Sim will then promptly leave the lot and attend the first rehab session for a few hours. On return he or she will have a buff informing you that the rehab is well underway. Once this buff runs out, you can click on the Sim again and start rehab session number two.
Same procedure goes for the third and final rehab session. Once all three sessions are done the addicted Sim will be rid off all addictions. Each session costs 1000 Simoleons.
If your Sim is drunk or high around their guardians (parents, aunts/uncles, grandparents) the guardians might bust them and yell at them. If they do this a second time the guardian will threaten them with rehab. If they do it a third time the guardian will send them straight to rehab regardless of the teen being addicted or not. All of these interactions will cause a massive relationship loss between the two parties involved.
If the teen follows through with all the rehab sessions the guardian will commend them on being sober and doing a good job and the two parties involved will get a massive relationship boost.