Addiction to a drug results in withdrawal symptoms when a person stops taking the drug. Coming off heroin can be a shattering experience.A heroin user finds that they have to increase the dose to have the same effect. The body has developed tolerance to the drug. The drug addict may then start using high amounts to stop the withdrawal symptoms and this gets to a level where they have too much for their body to cope with. This is when they are likely to take an overdose.