Welcome to HCM and thanks for posting your query here.
Sorbifer durules are iron and vitamin c
containing preparations. It is used in iron deficiency
anaemia and disorders related to iron deficiency. The iron absorbed with in the gastrointestinal tract is metabolised and is excreted in the stools. This will cause the stools to turn black. Actually it is a sign of working of the tablet within the body. Hence you no need to worry about that.
One more common side effect of the iron preparation is either diarrhoea
. In your case a stool softener
can be used to relieve the symptoms.
Hope this is helpful to you.