Question Summary:
In principle, it is impermissible for one to consume rennet derived from unlawful and Haraam animals

Question Detail:

i wanted to know about baby formula milk. I contacted all the companies who said that they 'use animal rennet in the process' of the formula but this rennet is not an ingredient and is not found in the final product. There is only 1 alternate vegetarian milk however these cause my baby constipation and there are no other products to use for such a young baby. Is there an allowance to use such a product? 

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh
In principle, it is impermissible for one to consume rennet derived from unlawful and Haraam animals such as pig. However it is permissible for one to consume rennet derived from lawful and Halal animals. You should contact the company to ascertain the source of the animal rennet. If the rennet is derived from unlawful sources, there is no other viable alternative and an upright and just Muslim health practitioner prescribes that product, then it will be permissible for one to consume such a product.
 
And Allah Ta’āla Knows Best
 
Ismail Desai,
Darul Iftaa
Checked and Approved,
Mufti Ebrahim Desai.