Generics aren't always chemically identical. The active ingredient might be the same, but the binders may not be. In the majority of cases, that's fine - you only really care about the active ingredient - but in certain cases, such as a sensitivity to one of those ingredients, it might not be. My sister has had to have specific brands of certain medicines prescribed due to a sensitivity to certain types of sugars. For instance, compare Neurofen:
> Each tablet contains Ibuprofen 200 mg Also contains: Sucrose, Sodium Citrate, talk, Croscarmellose Sodium, Stearis Acid, Titaniu, Dioxide, Silicon Dioxide, Acacia, Carmellose Sodium, Sodiu, Laurilsulphate, Macrogol and Blank Ink(contains Shellac, Iron Oxide Black (E172) and Propylene Glycol) or Opacode S -1 -8152 HV Black (contains Shellac, Iron Oxide Black (E172, Antifoam DC1510 and Soya Lecithin
> Each tablet contains Ibuprofen 200 mg Also contains: Sucrose, Sodium Citrate, talk, Croscarmellose Sodium, Stearis Acid, Titaniu, Dioxide, Silicon Dioxide, Acacia, Carmellose Sodium, Sodiu, Laurilsulphate, Macrogol and Blank Ink(contains Shellac, Iron Oxide Black (E172) and Propylene Glycol) or Opacode S -1 -8152 HV Black (contains Shellac, Iron Oxide Black (E172, Antifoam DC1510 and Soya Lecithin
https://www.boots.com/nurofen-200mg-tablets-16-tablets
to generic ibuprofen:
> Ibuprofen 400mg, sodium starch glycollate, croscarmellose sodium, sucrose, E171, E127 and E211
https://www.chemistdirect.co.uk/ibuprofen-400mg-tablets/prd-...