This is one of a series of questions dealing with the magic explained below:
In my fantasy world, magic is an energy similar to radiation. It alters the DNA of living cells, causing them to mutate or die. There are people in this world capable of controlling the magic. They can control the change, using magic either to kill cells, or change them in a host of beneficial ways. For example, they might give themselves night vision by altering their own eye cells.
This particular question deals with the repercussions of the above-mentioned magic users playing with the chemical functions of human cells. Assuming that they can alter a cell to control/halt its production of a chemical, what could they do? What would the limits be?
The users can cause the cells to increase, decrease, or halt production of a chemical. Also, if it is scientifically possible, they can alter the cells DNA sufficiently so that it produces different chemicals. Note that these chemicals are real chemicals which are really produce-able by cells.
Note that they are altering cells, not creating new ones or outright controlling the cells. These changes are slow, natural alterations - aside from the fact that they are prompted by magic-users. All the user is doing is altering a group of cells, perhaps over a wide area, and then leaving. The actual rate of production of chemicals is entirely up to the cells. The cells are left to multiply and spread the magical alterations naturally.