Text input field with multiple options

I have a medical app showing 3 input fields for drug dosage calculation. The middle field has 2 options, either to input the number of drug's ampules (as shown in the image), or to input thenumber of drug's milligrams. My current approach is to make the user click on the word "Ampules" to change it to "Milligrams", however the small arrow doesn't seem to be an enough indicator that this is possible. Besides, I'm considering to add a tooltip at the first run of the app as a tutorial for such behaviour. Can any one recommend me a better approach ?