A number of scientific and medical reviews have documented that the earlier anti-cannabis arguments of cannabis being a 'gateway drug' are flawed. The problem is that something that precedes another thing cannot be reasonably considered to cause the later thing. Drinking milk as a baby cannot be considered causal for heroin use, as an obvious example. Cannabis use and cocaine, heroin, meth, or other drug use are much more closely tied behaviors, but the simple fact of a relatively safe behavior preceding a more dangerous behavior is not sufficient to blame the safe behavior (smoking cannabis) as the cause of the later, dangerous behavior (injecting heroin).