please can have some help?

How does a decrease in the price of a substitute good/service affect the demand curve for the original good/service? Think of a healthcare example (i.e. what might a patient substitute for a prescription medication? What might a patient substitute for an emergency room visit etc....)? Do you think the resulting shift would have a positive or negative effect on the quality of healthcare (this part is subjective, so as long as you justify your opinion clearly, you will get credit)