Substance-induced mood disorders are caused by a chemical imbalance in the brain when chronic use of drugs or alcohol leads to changes in the brain’s neurotransmission to compensate for the constant changes and intensity during intoxication.

Some drugs mimic neurotransmitters. Heroin and prescription opioids, for example, chemically resemble the brain’s natural opioids (endorphin and enkephalin) sufficiently to engage and stimulate their specialized receptors. Since heroin stimulates many more receptors more strongly than the natural opioids, the result is a massive amplification of opioid receptor activity causing extreme or erratic moods. Marijuana mimics cannabinoid neurotransmitters, the most important of which is anandamide (also known as the “bliss molecule”) therefore causing high feelings of bliss, euphoria, and joy while under the influence, followed by extreme lows afterward. Nicotine attaches to receptors for acetylcholine, the neurotransmitter for the cholinergic system (parasympathetic nervous system).

Other drugs alter neurotransmission by interacting with molecular components of the sending and receiving process other than receptors. Cocaine, for example, attaches to the dopamine transporter, the molecular conduit that draws free-floating dopamine out of the synapse and back into the sending neuron. As long as cocaine occupies the transporter, dopamine cannot re-enter the neuron. It builds up in the synapse, stimulating receiving-neuron receptors more copiously and producing much greater dopamine impact on the receiving neurons than occurs naturally. This is significant because dopamine helps a person experience pleasure. If the brain’s natural ability to create dopamine is altered, the addict may be unable to feel any kind of pleasure, contentment or joy in daily activities for ongoing periods of time.