There are many psychoactive substances whose abuse can put the abuser and those in his immediate vicinity in great danger. Because of the myriad risks associated with drugs, as well as the wide variety of effects that any one substance can have on different individuals, it is impossible to pinpoint one substance of abuse which carries the greatest danger for a given user or his community; however, there are certain substances which put more people in more danger than others.
Different sources of information will nominate different substances as the universal worst. However, there are four substances which carry the greatest stigma: heroin, freebase (crack) cocaine, methamphetamine, and phencyclidine (PCP). Each of these substances carries titanic risk for anyone involved in their production, distribution, and use. Additionally, even the friends, family, and neighbors of users of these substances are in danger! These dangers come in many forms:
Direct overdose is the most immediate danger for the drug user. Heroin is the most dangerous substance of abuse in this regard. This is due to the inconsistency of its purity, the rapid increase in tolerance that the user experiences, and the most common route of administration. In order to understand why these conditions pose such great risks, let us examine a hypothetical scenario:
An intravenous heroin user buys a gram of black tar heroin. Most of the heroin that he buys is only around 10% pure. Because he has been using heroin frequently, he currently requires an entire gram of 10% pure heroin for the desired effects. However, this batch is more pure than what this user is accustomed to; it is 30% pure.
The user injects the entire gram of heroin into his arm, overdoses, and dies. Had he used a substance with more consistent purity, such as crack cocaine, he would not have ingested more of the substance than intended. Had he used a substance to which one builds a tolerance more slowly, such as PCP, he would not have intended to ingest as much as he did in the first place. Had he used a substance which is typically smoked or insufflated, such as methamphetamine, the drug would not have been introduced into his system as rapidly as it was.
Overdose is possible in any of these cases; it is simply not as likely with crack cocaine, methamphetamine, or PCP as it is with heroin.
However, there are many dangers associated with drug use other than direct overdose. For example, PCP reduces sensation and inhibition in users, which can lead to self-mutilation and violence toward others. Crack cocaine can cause cravings so intense that users will do anything for their next hit, including committing violent crimes. And methamphetamine can cause psychosis in users, which can lead to self-mutilation, high-risk behavior, or the deliberate infliction of harm onto others.
There is no single substance which can be clearly selected as the most dangerous substance of abuse. However, heroin, crack cocaine, methamphetamine, and PCP are four substances with the most risk involved. There are many, many other substances with which grave danger is associated. Any substance which changes the way a user feels carries risk of addiction, violence, and overdose.