Dependence occurs when, after a constant supply of the opiate, the brain shows adaptation, or a change in its circuitry. When that drug is taken away, neurons that have long been inhibited start pumping out neurotransmitters again. Tolerance, another poorly understood phenomenon, describes the need for a drug user to administer larger and larger doses of the drug to achieve the same psychoactive effect. A general hypothesis says that when the body's chemical equilibrium is upset, as in habitual drug-taking, the body sets up oppositional processes to restore itself. More of the drug is needed to overcome these efficient corrective processes.
Tolerance occurs with regular use of almost all psychoactive drugs.
It is not surprising that other drugs of abuse mimic or interfere with naturally-occurring neurotransmitters as well. Nicotine for example mimics a common neurotransmitter in the brain called acetylcholine which affects skeletal and heart muscle function. Cocaine and amphetamines cause an abundance of another neurotransmitter, dopamine, to stay in the cleft between nerve cells, stimulating them. The active ingredient in marijuana, THC, is mildly hallucinogenic and probably interacts with many chemical pathways. The opiates like heroin are the drugs that cause the most classic set of withdrawal symptoms.