Background: Injection drug use (IDU) is one of the most dangerous and addictive methods of drug abuse. The current statistics showed that the rate of IDU is increasing, and is higher compared to other methods of drugs abuse. In addition, it is one of the main routes of transmitting infectious diseases, such as human immunodeficiency virus (HIV) and hepatitis in Iran. Objectives: In light of this, the present study aimed at examining the main factors, affecting the tendency for IDU. Patients and Methods: This study was a descriptive study of 823 individuals, having an active file at centers and referred to Kermanshah-based maintenance methadone therapy (MMT) centers, in 2015. Sample collection was performed through multi-stage sampling of all MMT centers. Information gathering was performed with the help of a Psychologist of the ward, in charge of interviewing the referees. The data were analyzed, using multi-variable logistic regression. Results: All the clients were males with an average age of 38.48 ± 11.21 years. Average age of drug abuse initiation was 23.03 ± 7.9 years. After controlling for confounding variables, the individuals, who were a drug addicts for more than 10 years had 3.01 (1.6 to 5.6) times greater tendencies for intravenous drug addiction, compared to those, who were a drug addict for less than five years. In addition, individuals aged more than 35 years had 0.06 (0.01-0.22) times less tendencies to IDU drugs, compared to those, who were younger than 19 years old. Although the type of drug that the patient used for the first time, was not affecting the tendency, the risk of intravenous drug addiction in those, who used oral drugs was 1.74 (1.1 to 2.56) times more than those, who developed a drug addiction through nasal drugs. Conclusions: By taking into consideration that drug abuse initiation and the duration of drug use were the main causes of the tendency for IDU, prevention by decreasing the age of the development of drug addiction and decreasing the duration of drug use, could be an effective measure to decrease the rate of IDU in the society.