Ask the schools, but here's just a sample:
Letters of Recommendation | MD University Program | Case Western Reserve University
Will we accept letters of recommendation from Graduate Students or Graduate Teaching Assistants in my science courses?
While this is not the preferred method of obtaining a letter of recommendation, we will accept a letter of recommendation from a graduate student or graduate teaching assistant if it is co-signed by the course instructor or professor. We recommend that you obtain an additional science letter from another professor/instructor from another class, if possible, to completely satisfy the science letters requirement.
Admissions and Application Process - M.D. Program - Mayo Clinic Alix School of Medicine - Mayo Clinic College of Medicine & Science
We require at least one letter to be written by a science professor. A letter from a teaching assistant is also acceptable.
Admissions | Medical School | Medical College of Wisconsin
If the letter-writer is a Teaching Assistant, the letter must be co-signed by the Professor of the course.
HSDM requires three letters of recommendation and will accept a maximum of four. At least three of these letters must be from faculty members and at least two of the letters must be from science professors. Alternatively, a composite letter of recommendation from a pre-health advising committee...
hsdm.harvard.edu
Letters of recommendation should be written by professors who know the applicant in an academic setting: either they have taught a course that the candidate has taken or they have overseen the applicant's academic work in some way as an academic advisor, research mentor, or thesis mentor. Applicants should be confident that professors are able to write a personalized letter and are in a position to address the candidate's academic abilities. Oftentimes, professors and teaching assistants will collaborate in letter writing for students in larger lecture classes; HSDM will accept coauthored letters.
Welcome to Ohio's only state-supported dental school. The Ohio State University College of Dentistry has embraced its public purpose of educating exceptionally capable and compassionate dental hygiene and dental professionals, providing care to patients, conducting cutting-edge research, and...
dentistry.osu.edu
Can my TA write a letter of recommendation?
Yes, as long as the TA can have the professor sign off on the letter of recommendation.
Course credits A minimum of 87 semester credits or 130 quarter credits of courses from an officially accredited U.S. or Canadian college or university is required. A maximum of 64 semester credits from community or junior colleges will be applied to the 87 semester credits. (One semester credit...
dentistry.umn.edu
Two (2) letters from science or math professors who taught you in a lecture or lab course. Letters from teaching assistants are also acceptable.
Requesting Letters of Recommendation - UCI BioSci Office of Diversity, Equity and Inclusion
While technically you can, it is generally a rule to have letters written by professors rather than graduate teaching assistants (TAs). The professor may be in a better position to evaluate you and to compare you to current and previous classes of students. While TAs will often write fine letters and frequently write parts or all of letters which professors sign or co-sign, having a letter written and signed only by the TA may not do much to help your case. That said, it is better to have a strong letter from a graduate student than a letter from a professor that says little or nothing. Ultimately, because some schools specifically state that they will only accept letters from professors, it is in your best interest to get to know your professors well enough so that they can write a strong recommendation letter for you. If it’s absolutely necessary to get a letter from a graduate student, strategize with the student to have them draft the letter, ask the professor to edit and co-sign, ensuring that the language used is reflective of the joint opinion (e.g. using “we” instead of “I”). If you are doing research in a lab, a professor may ask your graduate student or postdoctoral supervisor to provide some input into the letter but they ultimately should be the one signing or co-signing it.