Answer by taking a quote from the following article:

Panic! at the Disco is an American rock band from Las Vegas, Nevada, formed in 2004 and featuring the current lineup of vocalist Brendon Urie, accompanied on tour by guitarist Kenneth Harris, drummer Dan Pawlovich, and bassist Nicole Row. Founded by childhood friends Ryan Ross, Spencer Smith, Brent Wilson and Urie, Panic! at the Disco recorded its first demos while its members were in high school. Shortly after, the band recorded and released its debut studio album, A Fever You Can't Sweat Out (2005).

After the Vices & Virtues tour cycle, Urie, Smith, and Weekes began writing and preparing for a fourth album. During the recording of the album, touring guitarist Ian Crawford, who joined the band in 2009 after the departure of Ryan Ross and Jon Walker, left the band citing his desire to make "real, genuine" music. On July 15, 2013, the album was announced as Too Weird to Live, Too Rare to Die!, with a scheduled release date of October 8, 2013. The first single, "Miss Jackson", was released on July 15, 2013, along with its music video to promote the album. Panic! at the Disco opened for Fall Out Boy on the Save Rock And Roll Arena Tour with Kenneth Harris replacing Crawford.  Shortly before the band began its first tour in support of the album, Smith wrote an open letter to fans regarding his abuse of alcohol and prescription medications since the recording of Pretty. Odd. Although Smith joined the band for the first handful of dates, he left the tour to "continue fighting addiction". Urie posted on the band's official website on August 7, 2013, that "It's become evident that Spencer still needs more time to take care of himself. I can't expect him to be fighting addiction one minute and be fully immersed in a national tour the next. With that said, the tour will continue without Spencer while he is away getting the help he needs." Since Spencer's leave of absence, Dan Pawlovich of the band Valencia has filled in on tour.  In an interview with Pure Fresh on September 23, 2014, Urie stated that he had already thought about ideas on the fifth studio album; however, he was not sure if it would be a Panic! at the Disco album, or a solo album. Urie has also stated there are no current plans for Smith to return to the band.

When did his addiction start?
his abuse of alcohol and prescription medications since the recording of Pretty. Odd.