what would be the effect of adding fluorine to doxycycline?

Some antibiotics have fluorine. Doxycycline is non-beta lactam and does not have fluorine.

There are only a few antibiotics with fluorine. Below are two:

risperidone and paliperidone comparison

They are both used to treat schizophrenia. The only difference is the alcohol group on paliperidone. I wonder what effect a methyl group in the same place would have. 

what would be the effects of fludrotestosterone or fludroestrogen?

Fludrocortisone and dexamethasone are corticosteroids already in use. 

Below are some un-fluorinated corticosteroids:

clozapine and tetracyclic antidepressants

It is an atypical antipsychotic. 

Tetracyclic antidepressant - Wikipedia

carbamazepine and tricyclic antidepressants

Is there a relationship? Carbamazepine is used to treat epilepsy. 

Carbamazepine - Wikipedia

Tricyclic antidepressant - Wikipedia

aciclovir, ganciclovir, valganciclovir, and valaciclovir comparison

All are antivirals. What would be the effect of replacing an alcohol group with a fluorine or a chlorine atom? 

chlorambucil - a melphalan cousin

It is a chemotherapy drug and has structural similarities with several others. 

Derpetology: melphalan, ifosfamide, bendamustine, and cyclophosphamide comparison