Barrier contraceptive methods include external and internal condoms, the diaphragm, cervical cap, contraceptive sponge, and vaginal spermicides. As a group, barrier methods have relatively high failure rates and are considered tier 3 contraceptives. The difference between typical use and perfect use failure rates for barrier methods is relatively large because the method is generally applied just before or at the time of coitus, and is highly user-dependent. To improve effectiveness, most methods can be combined; both external and internal condoms individually may be used in combination with another barrier method or higher-tier contraceptive method to concomitantly reduce STI transmission and improve effectiveness. The exception to this is the use of nonoxynol-9 spermicide in patients at high risk of HIV, as the spermicide can increase the risk of HIV transmission. External and internal condoms also should not be used together as there is a risk of displacement of either or both condoms.