Heroin is a dangerous enough drug on its own.  Mixing it with another drug can cause even further problems and significantly increases the risk of something going wrong. Mixing heroin with alcohol and cocaine can be especially problematic (dangerous) and should always be avoided.

Mixing heroin with alcohol is extremely dangerous. Both of these substances are depressants and when taken together can depress the pulmonary system so much that the person dies. Heroin and alcohol should thus, never be used together.

Another dangerous mix is heroin and cocaine. This is known as a speedball. Mixing such a strong stimulant and powerful depressant is bad on the heart. The depressant signals it to slow down and the stimulant to speed up. An arrhythmia may occur, which can lead to death. John Belushi, a famous comedian and musician (of Blues Brothers fame) died after taking a speedball. River Phoenix, a promising young actor, died from this as well. 

In summary, heroin use is quite dangerous and it is very risky to mix it with other drugs. Some increase the likelihood of death, while other mixtures are simply unpleasant. Amongst the worst combinations include heroin and alcohol and heroin and cocaine. Heroin and alcohol are both central nervous system depressants. If too much of either drug is taken independent of the other, there is the risk of dying from respiratory depression. When taken together, this risk increases significantly, along with the potential for death. 

Heroin taken alongside cocaine is known as a speedball. This very powerful cocktail is fairly popular. This is unfortunate because it is quite dangerous. Cocaine and heroin have differing effects on the heart. Cocaine, because it is a stimulant, will cause the heart to speed up. Conversely, heroin, a depressant causes the cardiovascular system to slow down. This is very straining on the heart. It may even lead to the heart skipping a bit, arrhythmia, which can be fatal.

Not all drugs are dangerous when used with heroin. Tobacco isn’t. People who smoke and use heroin, typically don’t face any immediate threat. Of course, long term use of both is unhealthy and can lead to the development of diseases, such as cancer. However, unless a person overdoses on heroin, tobacco won’t cause any immediate risk to the user.