Background: Heroin is an extremely addictive narcotic drug derived from morphine. Its continued use requires increased amounts of the drug to achieve the same effect, resulting in tolerance and addiction. This study was done in order to determine the prevalence of musculoskeletal pain and forward head posture among heroin users during their withdrawal. Methods: This research was a cross-sectional study that was done on 90 heroin users (83 males, 7 females) aged between 20 to 40 years (32.5 ± 3.81) during their withdrawal in Shiraz, Iran. They were selected by simple randomized sampling. Data were collected by a form regarding age, sex, the duration of heroin use, and musculoskeletal pain. Pain was measured by VAS (visual analog scale) and forward head posture was evaluated by plumb line. Pearson correlation technique and chi-square were used for analyzing the data. Findings: The results revealed that the majority of heroin users suffered from musculoskeletal pain during their withdrawal. At the end of withdrawal 53.4% had severe pain, 38.8% had moderate pain, and 7.8% of them had mild pain. Pain in the lower extremities and low back was more common than the upper extremities. The intensity of pain before withdrawal was mild, during withdrawal was moderate, and at the end was sever, but there was no significant correlation between them. The results also showed 43.3% of subjects had normal posture and 56.7% had forward posture. Conclusion: According to the results, the intensity of pain increased during the withdrawal period; therefore, more attention must be paid to this complication in heroin users for better evaluation and a successful withdrawal.