Thanks for writing.
Doxycycline 100mg taken 2x day for 14 days is an effective cure for primary Syphilis.
Doxycycline is the second-line treatment for primary Syphilis. The first-line treatment is Bicillin (a form of penicillin), but Doxycycline is used when clients’ have an allergy to penicillin.
Syphilis is not resistant to Doxycycline, and will not become resistant during the two weeks of medication.
Sometimes it can take longer for the Syphilis sore to heal with Doxycycline, as the medication is taken over two weeks (instead of one dose up front). However, the Doxycycline will heal the Syphilis sore and stop incubation and transmission.
It takes two weeks for the Doxcycline to completely kill the Syphilis, so you need to not have sex while taking the medication, or you can pass it to your partners.
Hope this helps. Please feel free to submit another question as needed.