Tiki and Ghost Apple have been friends and occasionally neighbors ever since they were children. Tiki would temp live with his mother Twilight Sparkle in Ponyton for three weeks, then spend the fourth week in Canterlot with his other mother Celestia. He always came back to Ponyton in a snappy mood.
To alleviate his mood the pair would smoke weed, snack, and talk. Sometimes engaging in one of Tiki's favorite activities, finding typos in books.

Celestia doesn't concern herself with her son so the weed is never an issue to her. It's never been illegal to smoke marijuana in Equestria, it's viewed as similar to alcohol. Moderation is important. While Ghost only partakes occasionally, Tiki smokes twice a day everyday. It's getting excessive and Twilight is growing concerned. Tiki feels she's overreacting, even touting the drug's various 'health benefits'.

It's not like Twilight can do anything about it right? Pfft. We'll see about that.