The use of amphetamines, such as speed and ecstacy, in teens could lead to longer lasting depression later in life. "Researchers in Canada interviewed 3,880 teenagers from low-income neighborhoods [...]. Compared to their peers who used neither drug, teens who reported taking [drugs] at least once in the tenth grade had 70% and 60% higher odds, respectively, of experiencing depression symptoms in the eleventh grade." While this study cannot determine causality (in other words, say the drugs and depression have a cause-and-effect relationship), other influential factors, such as prior history of depression and problems in school/at home, were controlled for. The basic gist of the article is that studies have shown there to be some kind of link between amphetamine use in teens and depression later on.
The ethos of the researchers was affected by several factors. They increased their ethos by acknowledging the possible influence of other factors. They were not completely successful in controlling for alcohol and marijuana, which means there may be an association there. They decreased their ethos, however, in another respect. The study focused on the early teen years and the start of experimentation among these teens; the study did not focus on older teens' drug use, so the study could not comment on depression among this age group.