I personally got bitten by a tick, did the prophylaxis dose, and then developed joint pain which continued until I did a full 21 day course. Yeah it sucks not being able to go in the sun for an entire month and having acid reflux, but imho the evidence in favor of the prophylaxis is extremely dubious.
source: https://www.nejm.org/doi/full/10.1056/NEJM200107123450201
"A theoretical risk associated with prophylactic antimicrobial treatment is that it might alter the disease presentation so that the characteristic erythema migrans rash might not be manifested in treated subjects, in whom a more subtle, nonspecific illness might develop or asymptomatic seroconversion might occur. In such circumstances, an unrecognized latent infection might eventually result in arthritis or neurologic disease. We believe that this is unlikely for several reasons. [...]"