Opioid medications do their job well, and they do so by turning on something called our opioid receptors, which are found in the brain as well as in the gastrointestinal tract and spinal cord. The opioid receptors help signal and communicate about pain in the body, and both the opiate-like substances our body makes itself (endogenously-produced opioids) and the pills we take (exogenously-administered opiates) activate these receptors.
However, what happens when the body's opioid receptors deal with long-term exposure to opioid agonists? Hormone levels begin to fall out of balance, with decreased levels of sex hormones including testosterone and estrogen, cortisol and dehydroepiandrosterone sulfate (DHEAS) circulating throughout the body. This is what recent research calls opioid-induced endocrinopathy.
Emerging research is finding that opioid usage begins to take a toll on the hypothalamic-pituitary-gonadal (HPG) axis, causing it to malfunction. What happens is that opioids bind themselves to opioid receptors in the body, and those that bind themselves in the brain (the hypothalamus) disrupt how the body produces and manages sex hormones (and the verdict is still out how the other two receptors play a role in this). The way things are supposed to work goes like this: The hypothalamus produces gonadotropin-releasing hormone (GnRH), which triggers the pituitary gland to produce a luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH and FSH trigger the reproductive organs to produce estrogen (in women) and testosterone (in men) -- this is the HPG axis at work. If the HPG axis isn't functioning properly, the results can mean big changes inside the body, including changes that may lead to opioid-induced hypogonadism, sluggish libido, irregular menstruation in women, erectile dysfunction in men, and both female and male infertility, among other problems [sources: Ballantyne, Sigman].
While the affects of opioids on fertility are not yet fully understood, current research suggests long-term opioid usage, including the use of prescription drugs such as Tramadol, may play a role in both male and female fertility.