Even though it might hardly ever occur to you that something may go wrong during anal sex, you should always get prepared before you start to have anal sex. You should know that anus is the opening of the colon – or more specifically, the rectum – which is about 1.5 meters long with about 4 liters in volume. It means that depending on the quantity of “food” inside not more than 4 liters – 1 gallon – of lukewarm water is needed to clean its entire length. However, you are not likely to insert almost 2 yards inside, and since the usually suggested volume is about 2 liters for an enema in a hospital, you should use about half a liter before anal sex. It should be plenty to clean out everything from the rectum, but it will never be fully emptied, since some residual bacteria will be found there, so even if it is harmful for the vagina not the anus, it should be kept in mind not to insert anything into the vagina without washing it after an anal action.
Lubrication is yet another key factor of healthy anal sex, since the rectum is not naturally lubricated. Ideally, water based lubricants should be used, but if there is nothing else, even body lotion will do, as long as it does not get into the vagina, which has a sensitive mucous membrane that can be hurt by perfumed agents. But if no lubrication is inside the anus, it can result in anal fissures which will take several weeks or even months to heal, while sex should be avoided.