I am a second year graduate student at a large research university. I am a Ph.D. student, but I have not yet taken quals. My field of research is Physics Education. Last semester I TA'd a studio-based introductory course of about 81 students (the first semester algebra-based course that pre-meds take), which I was not the instructor on record. The studio-based method allows for much interaction and very little lecture, so the duties of a TA vs. a professor are essentially the same. A student from this course has approached me and asked if I will write them a letter of recommendation for medical school. The student was very good and I would have no trouble writing a good LOR for them, but should I? I have advised the student that it would most likely be better to get a LOR from a professor whose reputation would far exceed mine. Should I write the letter and just have the professor on record co-sign? I am really not sure if a LOR from a TA would hold up very well against other applicants. Thanks in advance.