The disadvantages of insulin obtained from the pancreas of slaughtered cows and pigs are:
Before the advent of rDNA technology, insulin was obtained from the pancreas of pigs and cows.
However, there was a slight structural different of 1-3 amino acids between the animal insulin and human insulin. When his animal insulin was administered by a diabetic patient he developed antibodies against the animal insulin thereby causing allergic reactions.
Another disadvantage was large number of animals were to be sacrificed for extracting the insulin from their pancreas. For example: To obtain 5 kgs. Of pancreatic juice about 75 pigs have to be killed to get insulin for treating only a single diabetic patient just for one year.