Follow-up testing is recommended to be sure that treatment is successful. Having syphilis once does not protect a person from getting it again. Even following successful treatment, people can still be re-infected. Only laboratory tests can confirm whether someone has syphilis.
Because syphilis sores can be hidden in the vagina, anus, under the foreskin, or mouth, it may not be obvious that a sex partner has syphilis. Unless a person knows that their sex partners have been tested and treated, they may be at risk of getting syphilis again from an untreated sex partner.