Pre-ejaculate (also known as pre-ejaculatory fluid or Cowper's fluid) is the clear lubricating fluid that is issued from a man's penis when he is aroused. Slang terms for pre-ejaculate include pre-come, pre-cum, dog water, and speed drop.
The fluid is usually secreted by Cowper's glands during foreplay or at an early stage during sex, some time before the man reaches orgasm and semen is ejaculated. Pre-ejaculatory fluid prepares the urethra for the passage of semen and neutralizes the pH of any residual urine. It also lubricates the movement of the foreskin over the glans. The amount of fluid that the human male can issue varies widely between individuals, from imperceptible amounts to a copious flow.
A problem for couples who seek to avoid pregnancy, however, is that the pre-ejaculate may contain sperm. This is one additional reason why withdrawal (coitus interruptus) is not considered to be a reliable method of birth control. However, some research has been done which suggests that there are no live sperm. In fact studies have so far shown that pre-ejaculate contains only "small clumps" of non-motile sperm at rates well below the levels necessary for fertility.[1]
A more serious problem is those same studies have shown the presence of HIV, the virus responsable for the disease AIDS, in pre-ejaculate. Although the frequency of HIV transmission through oral-genital contact is fairly low [2] a risk still exists.