Prolactin (PRL) is a hormone produced by the pituitary gland. 

It is normally associated with lactation in breastfeeding women, but men, and women who are not pregnant also have prolactin.

Research has shown that pituitary hormone prolactin (PRL) plays a role in regulating sexual satisfaction for both men and women. 

From a practical standpoint, prolactin appears to correlate with the feeling of satiety. This objectively makes sense, as prolactin follows dopamine release, so after eating a large meal, or having good sex, or drinking a good beer, the feeling of satisfaction occurs.

Effects of long-term elevation in prolactin levels

High levels of prolactin in men over sustained periods can impact men in many ways;

• Loss of libido
• Mood changes/depression - partularly the feeling of hopelessness 
• Impotence
• Headache
• Infertility
• Decreased testosterone levels
• Weight gain
• Peripheral vision problems
• Gynecomastia (growing breasts)

Causes of elevated prolactin

Elevated dopamine levels 

Prolactin is thought to significantly elevate in response to high dopamine. In simple terms, anything which significantly spikes your dopamine - orgasm, gambling, alcohol, fast foods, and some recreational drugs - will likely follow with a significant release of prolactin.

The theory is that the two are inter-related. Dopamine motivates you to pursue something with your full attention, such as sex. Prolactin steps in and puts on the brakes, to return your mind to its normal state. Stop eating so you don't explode. Stop having sex and pursue other useful things like food and sleep.

Sex, orgasm and ejaculation

After mating, female rats show surges in prolactin for up to two weeks—even if they don't get pregnant. 

In a study of human females;

Women who had orgasms saw increases in their prolactin levels, with the number of orgasms affecting the increase. The greatest surges occurred in women who had two orgasms. However, women who had no orgasm had no increase. In fact, their prolactin levels were lower after sexual activity than they were at baseline. [ more... ]
- International Society for Sexual Medicine

Recreational drugs

For example, cocaine blasts the brain with high levels of dopamine, and prolactin rises during withdrawal. Indeed, addicts going through withdrawal required two weeks for their prolactin levels to drop to normal. 

Further Reading

Study - The post-orgasmic prolactin increase following intercourse is greater than following masturbation and suggests greater satiety.

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