I cannot stress how important it is to get letters of recommendations from physicians you have shadowed or worked with. These letters provide proof that you did in fact shadowed a physician instead of just taking your word for it since you wrote it down on your application that you did indeed shadow.  In addition, the physician that is writing your letter is a "colleague" of the physician reading your application. Although, they will not know who they are per se, they will take into consideration their colleague's professional word.

Hopefully the letter will be able to provide metrics (i.e. the number of months you spent shadowing, number of hours per week shadowing, etc.).  The most important aspect of the letter is to provide commentary on your professionalism in a clinical setting. Are you always at the physicians office on time?  Are you always well dressed? Did you have a good demeanor or personality at the clinic? Did you get along with the patient, nurses and even the secretarial staff? Were you able to ask intelligent questions about certain diseases, etc.?

After your clinical experience or near the end of your clinical experience, kindly ask them for a letter.  If they agree, then provide them with your letter of recommendation packet (personal statement, stamped envelopes, resume, etc.) A letter from a physician commenting on how you were in a clinical setting is a critical letter for your application packet.