IMO, the purpose of the letter is two-fold. First, they want to hear from someone who is an objective (keep in mind you can't read these letters) third party who can let them know you are a good student (whether in school, at a pharmacy, or in another learning capacity.) While anyone within the limitations placed by the school (generally not family/friends) can write the letter, what subjects do you want the school you know that you do well in? If you do well in glassblowing, what does that tell the pharmacy school about your ability to learn chemical interactions, homeostatic adjustments, etc? Second, they want to know how you get along with other professionals. What relationship do you have with your professors, pharmacist, managers at work (where allowed)? This is why it's important to have the letter written by someone who knows you well. They have to know that you are willing to approach faculty to ask questions when it's important, that if you don't understand a concept you aren't too timid and would rather be ignorant than informed. I'd have them written generally by math/science professors since these are the closest to the subjects pharm schools need you to be good at.
If you want to have one written by a speech/English professor to show your communication skills (ESL/international student) I'd say do that. Otherwise, I'd stick to sciences. As mentioned above, pharmacist trumps all.
