Some say sex is the most powerful drug in the world. It does excite the same brain areas as amphetamines - and makes male rats more prone to seeking out drugs when the times of daily sex are over.
Sex can be addictive. The pleasure you feel during sex might make some people want to have sexual intercourse all the time, again and again.
If that wasn't enough, it can make you even more susceptible to other - chemical - drugs.
Rats who usually mate everyday tend to seek out designer drugs like amphetamines much more often during abstinence than they normally would.
Now a team of US-researchers led by Lauren Beloate at the University of Mississippi Medical Center in Jackson have found out what causes this phenomenon.
The study was published in "The Journal of Neuroscience".
Both sex and amphetamines act in the same neural pathways in the brain, the so-called ventral tegmental area, the researchers found.
This group of neurons produce the neurotransmitter dopamine and are involved in the natural reward circuit of the brain.
To scientists like Beloate and her team, sex is nothing more than a natural reward: it makes us feel good, driving the desire to repeat this activity soon.
Drugs act on the same part of the brain, which makes them so addictive.
"Experience with sexual behavior, followed by a period of abstinence from sexual behavior, causes increased reward for amphetamine in male rats," Beloate's team writes.
Having sex every day altered the rats' brain, making it more prone to drug use.
Sexually experienced rats became addicted to amphetamines faster and at much lower doses than rats who didn't mate.
The scientists call it an "increased vulnerability," meaning that the drug has a much higher effect on sex-engaged animals than it would otherwise.
In rats, five days of sex in a row followed by a seven-day abstinence, was enough to trigger such a change in the brain.
A two-way street
Methamphetamine users report having heightened sexual pleasure and engaging in unprotected sex due to loss of control.
Scientists found in prior animal experiments that regurlar administrations of low doses of methamphetamine - also called meth - resulted in compulsive sex-seeking behavior during drug abstinence.
This effect, though, was only observed when rats had sexual intercourse right after methamphetamine administration.
Very high doses of meth, however, made rats unwilling to have sex at all.