Can I use this Ispaghula 3.5gm + Lactitol 10gm Syrup as a substitute for my prescribed medicine?

A substitute is only appropriate if it has the same active ingredients at the same strength, so it depends on what you were prescribed. If your prescribed syrup also contains ispaghula and lactitol in these amounts, this can generally replace it; if it has a different composition, it isn't a like-for-like swap. Please show your prescription to your doctor or pharmacist before switching.