Manforce 100 mg contains sildenafil 100 mg. Back pain can occur as a side effect in some men, although it is not among the most common effects. Sildenafil-related muscle or back discomfort may appear after taking the dose, often within a few hours or by the next day. Possible reasons include: Muscle aches or lower back discomfort as part of the body’s response to sildenafil. Changes in blood vessel activity and muscle relaxation, which can sometimes be associated with aches or discomfort. A higher dose (100 mg) may increase the chance of side effects compared with lower doses in some people. Usually, mild back pain: Starts within the same day or the following day Improves as the medicine leaves the body Does not require treatment if it is mild and short-lived Seek medical advice if: The back pain is severe .