When a doctor thinks that a patient needs medicine, they will write a note explaining which medicine, and how much to take, which the patient takes to a pharmacy. This note is called "a prescription". The doctor's action is expressed as "writing a prescription". Notice the different way this phrase can be used:

She wrote a prescription.

She wrote me a prescription.

She wrote me a prescription for some antibiotics.

She wrote a prescription for some antibiotics.

You can also use the verb "prescribed":

The doctor prescribed a pain killer.

But "prescribed" sounds a little more formal.