There is little conclusive scientific evidence about the long-term effects of human cannabis consumption. The findings of many earlier studies purporting to demonstrate the effects of the drug are unreliable and generally regarded as junk science, as the studies were flawed, with strong bias and poor methodology. The most significant confounding factor is the use of other drugs, including alcohol and tobacco, by test subjects in conjunction with cannabis. When subjects using only cannabis were combined in the same sample with subjects using other drugs as well, researchers could not reach a conclusion as to whether their findings were caused by cannabis, other drugs or the interaction between them. In addition, research using cannabis is heavily restricted in many countries, making it difficult to get new studies funded or approved. Since there are so many different compounds in cannabis, it is difficult to predict or accurately measure its effects.
Some conclusions established with some degree of certainty, however, are that cannabis is less likely to cause emphysema or cancer than tobacco; that sustained early-adolescent cannabis use among certain genetically predisposed individuals has an elevated correlation with certain mental illness outcomes, ranging from momentary minor psychotic episodes to clinical schizophrenia; that cannabis use is generally higher among schizophrenics, but causality has not been established; that it temporarily impairs motor skills; that it is unlikely to cause birth defects or developmental delays in the children of users, and in a study done by the University of California Los Angeles in 2006, that even heavy marijuana smokers do not increase their risk for lung cancer. This is most likely due to the active ingredient delta9-THC, which recent studies indicate could well fight cancer by causing cell death in aging and potentially cancerous cells in vitro.