When a person is physiologically dependent on a psychoactive substance, it is characterized by two factors:
1. Tolerance forming
2. Withdrawal symptoms.
First, tolerance formation refers to the phenomenon where prolonged use of a physically addictive substance causes a reduced reaction to it, leading users to consume larger quantities of the substance to obtain the desired effect. For instance, an alcoholic will likely need larger doses of alcohol consumption to obtain the desired effect than when he or she first started consuming alcohol.
Withdrawal effects on the other hand refers to unpleasant and often debilitating physical and psychological effects people suffer from after suddenly ceasing or drastically cutting down using a substance they have been consuming for a prolonged time period. Withdrawal symptoms can include headaches, nausea, vomiting, convulsions, low moods, etc.