Just Before Dawn.

In the 7th film of the "Crime Doctor" series based on the radio program, Dr. Robert Ordway is summoned to take attend a diabetic, and gives an injection of insulin taken from a bottle in the patient's pocket. The man dies and Ordway discovers that what he thought was insulin was really poison. Oops! Two other people are murdered before Ordway discovers who replaced the insulin with poison and what the motive was