Male infertility has dozens of possible causes, ranging from physical conditions and hormonal shifts to lifestyle habits, medications, and genetic factors. About half of all infertility cases involve a male factor, and in many of those, more than one issue is at play. Understanding the specific ways fertility can be disrupted helps clarify what’s reversible, what’s preventable, and what might need medical intervention.
Varicoceles and Overheated Testicles
Sperm production depends on temperature. The testicles need to stay roughly 2.2°C (about 4°F) cooler than the rest of the body. Even a 1 to 1.5°C increase can shrink testicular size, lower sperm output, and produce abnormal sperm. DNA synthesis inside the testicles is especially sensitive, with peak function around 31°C, well below normal body temperature.
A varicocele, an enlarged vein in the scrotum, is one of the most common treatable causes of male infertility. It works by impairing blood drainage from the testicle, which raises scrotal temperature and starves the tissue of oxygen. The combination of excess heat, low oxygen, and a weakened blood-testis barrier ramps up oxidative stress, damaging the cells that produce sperm. Varicoceles are found in a large share of men evaluated for infertility, and surgical correction often improves sperm counts.
Occupational heat exposure creates similar problems. Men who work near furnaces, in bakeries, or in other high-temperature environments face sustained scrotal heating that can suppress sperm production over time. Prolonged sitting, frequent hot tub use, and keeping a laptop on your lap all raise scrotal temperature in the same direction, though the effects are generally milder and reversible once the heat source is removed.
Hormones and the Testosterone Trap
This is one of the most counterintuitive causes of male infertility: taking testosterone can shut down your sperm production. The testicles produce sperm using locally made testosterone at concentrations far higher than what circulates in your blood. When you introduce testosterone from outside the body (injections, gels, patches), your brain detects the rising blood levels and stops sending the hormonal signals that tell the testicles to produce both testosterone and sperm. The result is that sperm counts can drop to zero.
The good news is that this process is usually reversible. Once testosterone therapy stops, sperm typically reappear in semen within about three months, and no permanent damage is expected. But men who use testosterone replacement without understanding this risk sometimes discover it only when they’re trying to conceive.
Other hormonal disruptions matter too. Conditions affecting the pituitary gland, such as tumors or prior head injuries, can reduce the signals that drive sperm production. Thyroid disorders and elevated prolactin levels can also interfere.
Anabolic Steroids
Anabolic steroids hijack the same feedback loop as testosterone therapy, but often more aggressively. Men who use steroids for bodybuilding or performance frequently develop severely low sperm counts or produce no sperm at all while using them. Recovery after stopping is possible, but slow. Sperm production can take over a year to normalize, and some men need hormonal treatment to restore function. The recovery window ranges from 4 to 30 months, and a subset of men experience prolonged symptoms of low testosterone during that period.
Medications That Impair Fertility
Several common prescription drugs can reduce sperm count, motility, or quality. The effects vary by drug class:
- Hair loss medications (finasteride, dutasteride): These drugs block the conversion of testosterone into a more potent form. They can lower sperm counts and motility, and some sexual side effects may persist even after stopping.
- Antidepressants (SSRIs): Medications like sertraline and fluoxetine can suppress testosterone levels, reduce sperm counts, and may increase DNA damage in sperm.
- Opioid painkillers: Chronic use of opioids like oxycodone or methadone suppresses testosterone and can reduce sperm count, movement, and shape all at once.
- Chemotherapy drugs: Certain cancer treatments, particularly alkylating agents like cyclophosphamide, are among the most damaging to sperm-producing cells. The damage can be permanent, which is why sperm banking before treatment is standard practice.
If you’re planning to have children and currently take any of these medications, the timing of that conversation with your doctor matters. Some effects are reversible with enough time off the drug; others are not.
Environmental Chemicals
Endocrine-disrupting chemicals are now widespread in everyday products, and their effect on male fertility is increasingly well documented. Phthalates, found in plastics, food packaging, and personal care products, have been linked to lower total sperm counts, reduced concentration, and decreased motility. In lab studies, phthalate exposure directly impairs sperm’s ability to generate the energy it needs to move.
BPA, another plastic-derived chemical, shows similar effects. It’s detectable in the urine of most men tested, and exposure is associated with reduced sperm quality. Animal research has shown that BPA exposure even before birth, during fetal development, can lower sperm counts in adulthood. These chemicals mimic or interfere with natural hormones, disrupting the signals that regulate sperm production.
Pesticides, heavy metals like lead and cadmium, and industrial solvents round out the list of environmental threats. Reducing exposure is partly within your control (choosing glass over plastic, filtering water, avoiding unnecessary chemical contact), though complete avoidance is unrealistic given how pervasive these compounds are.
Physical Blockages
Some men produce sperm normally but have a physical obstruction preventing it from reaching the semen. This is called obstructive azoospermia, and it accounts for up to 40% of cases where no sperm appears in the ejaculate. Blockages most commonly occur in the epididymis (the coiled tube where sperm matures), the vas deferens (the duct that carries sperm), or the ejaculatory duct.
Causes include prior surgery (a vasectomy is the most obvious example), trauma or injury to the testicles, and infections. Sexually transmitted infections like chlamydia and gonorrhea can scar reproductive ducts if untreated. Men who carry certain cystic fibrosis gene mutations may be born without a vas deferens entirely, or develop thick secretions that block it. Because sperm production itself is normal in these cases, surgical repair or sperm retrieval for assisted reproduction is often successful.
Genetic Causes
Some men are born with genetic differences that limit or eliminate sperm production. One of the most studied is Y chromosome microdeletion, where small segments of the Y chromosome are missing. The Y chromosome contains regions specifically needed for making sperm, and the impact depends on which region is affected.
Deletions in the AZFa or AZFb regions cause the most severe damage, typically resulting in complete absence of sperm with no realistic option for sperm retrieval. Deletions in the AZFc region produce a wider range of outcomes, from mildly low counts to no sperm at all. Men with AZFc deletions often still have enough sperm for assisted reproduction techniques to work. Genetic testing helps determine which scenario applies and what options are realistic.
Chromosomal conditions like Klinefelter syndrome (where a man has an extra X chromosome) also impair fertility, as do certain single-gene mutations that affect hormone production or sperm development.
Age and Sperm Quality
Male fertility doesn’t have a hard cutoff the way female fertility does, but it declines more than most people realize. As men age, testicular function decreases, reproductive hormone levels shift, and sperm DNA becomes increasingly fragmented. This accumulation of DNA damage raises the rate of failed pregnancies and complications even when conception occurs. Older paternal age is also associated with lower success rates in IVF and related procedures, along with a higher risk of genetic disorders in offspring. The decline is gradual rather than sudden, but it becomes measurable in a man’s late 30s and more pronounced through his 40s and beyond.
How Infertility Is Measured
A semen analysis is the starting point for evaluating male fertility. The World Health Organization’s current reference values set the lower limits of normal at a sperm concentration of 16 million per milliliter, total motility (any movement) of 42%, and normal shape in at least 4% of sperm. Falling below any of these thresholds doesn’t guarantee infertility, but it indicates reduced fertility potential. Many men with borderline results conceive naturally; others with apparently normal numbers still struggle due to factors like DNA fragmentation that standard semen analysis doesn’t measure.
When semen analysis shows zero sperm, blood hormone levels and genetic testing help distinguish between a production problem and a blockage, which matters because the treatment paths are completely different.