android – Text input field with several options

I have a medical application displaying 3 input fields for calculating the dosage of the drug. The middle field has 2 options, either enter the number of ampoules of the drug (as shown in the picture), or enter the number of milligrams of the drug. My current approach is to have the user click on the word "Ampules" to change it to "Milligrams". However, the little arrow does not seem to be a sufficient indicator for this to be possible. In addition, I plan to add a tooltip when first running the application as a tutorial for such behavior. Can someone recommend me a better approach?

screen capture