When Tosh first joined Torchwood, Suzie would invite Tosh over to her flat. Suzie would soothe Tosh's fears of violating her UNIT agreement. Then they'd go to Suzie's flat and drink all evening.
At first, the drinks were potent enough to block out the horrors Suzie and Tosh faced while working at Torchwood. Over time, though, the drinks needed to forget work increased. Eventually both girls developed a tolerance.
After one session, Tosh realized she could die on the job. Suzie realized she might go crazy because of Torchwood.
They held each other's hands for dear life after that session.