Metformin is famous for diarrhoea; I had never heard of constipation before; any drug can cause any side effect, so I have learnt something new (I would never be patronising). Half folk on metformin find their gastrointestinal side effects resolve on switching to the modified release, which means that the other half do not, unfortunately.
Metformin is as "strong" as other tablets for diabetes, doesn't cause hypos or weight gain on its own, may decrease risk of heart attack or stroke, may improve lipids and may decrease risk of cancer, improve outcomes if develop cancer; so for a drug whose origin is in the plant Goat's Rue 500 years ago, it is amazing stuff!
best wishes