Syphilis is a common sexually transmitted infection. Penicillin is one of the oldest known antibiotic. The syphilis bacterium has not been shown to have developed resistance to penicillin in the past. So penicillin is very effective in the treatment of syphilis especially in pregnancy. Penicillin crosses the placenta and treats the baby in the womb better than most other known antibiotics. This ability of penicillin to cross the placenta makes it a very good treatment option for syphilis during pregnancy. One shot of the injection can cure the disease both for the pregnant lady and the baby in her womb. Penicillin in the right dosage can be given to non-pregnant women, babies and to men also, if they test positive for syphilis.