How long does it take for dexmethasone to work for back pain?

Steroids for LBP.: Assuming that the steroids will be beneficial for the actual mechanism of your back pain, they generally are kicking in en force within 24 hours.

12-48 hours: You can expect to feel some decrease in pain by 2 days afterward. But depending on the severity and type of your injury, it may not completely resolve your pain. You may need other modalities for healing, too, such as anti-inflammatories, muscle relaxants, physical therapy, massage, chiropractic, etc.