Rebecca Arnold, of Mornington Primary School, asks :-

How do doctors know what medicines to give patients for their illnesses and diseases?

Cynthia Darlington, a pharmacologist at the University of Otago Medical School, responded.

When a person visits their doctor they tell the doctor what they are feeling and why they have come to the doctor. The doctor then examines them to determine why the patient feels that way. The doctor will make a “diagnosis” based on all the information she gets from the patient’s description and from their examination of the patient.

Medicines are classified into different “drug categories” based upon the kind of illnesses they are used to treat. For example, antibiotics are used to treat infections caused by bacteria but not infections caused by viruses.

Once the doctor has determined the category of drug that is needed, she will have a number of drugs in that category from which to choose. She will consider how serious the illness is. If the illness is very serious, she will choose a very strong medicine from the drug category and the patient may need to take it for a long time. If the illness is mild, she may choose a less potent medicine and the patient may take it for only a short time. The doctor will need to know all the other medicines that the patient is taking, even medicines that can be bought in a supermarket or health food store. Some medicines cannot be used at the same time, and combining them can make the patient even sicker. The doctor will choose a medicine that does not have an ”adverse interaction” with any other medicines that the patient is taking. Sometimes the age of the patient makes a difference in the choice of medicine and the doctor will take that into consideration.

Finally the doctor will consider the “side effects” the drug may cause. Side effects are things that the drug does that are not part of the treatment for the illness. Sometimes the doctor may choose one drug over another based on the side effects. Other times the doctor may say, “I am going prescribe this drug for you. You need to know that this drug may cause thirst as a side effect. If you feel extra thirsty while you are taking this medicine, that’s OK. It is not a harmful side effect and we expect it to happen.”