A low sperm count has many possible causes, ranging from treatable medical conditions to everyday habits you might not suspect. The clinical threshold is fewer than 16 million sperm per milliliter of semen, or fewer than 39 million total per ejaculate. If your numbers fall below that range on a semen analysis, one or more of the factors below is likely responsible.
Varicocele: The Most Common Medical Cause
A varicocele is an enlarged vein inside the scrotum, similar to a varicose vein in the leg. While varicoceles are present in about 15% of all men, they show up in roughly 40% of men with fertility problems. The leading explanation is heat. Your testicles need to stay around 35°C (95°F), about two degrees cooler than core body temperature, to produce sperm efficiently. A varicocele impairs blood flow and raises scrotal temperature, which disrupts sperm production over time. Varicoceles are usually painless, so many men don’t know they have one until a fertility workup.
Hormonal Imbalances
Sperm production depends on a chain of hormonal signals that starts in the brain. The pituitary gland releases two hormones: one that stimulates the testicles to produce testosterone, and another that directly drives sperm production. If either hormone is too low, sperm output drops. Without adequate testosterone, the testicles can’t sustain the environment sperm cells need to develop. And even when testosterone levels are normal, a deficiency in the second signal (FSH) means the testicles aren’t fully stimulated to make sperm.
Conditions that disrupt these signals include pituitary tumors, genetic disorders, and certain chronic illnesses. Importantly, taking testosterone from an outside source (injections, gels, or patches) suppresses the brain’s signals and can shut down sperm production entirely. This is one of the most common medication-related causes of low sperm count.
Heat Exposure and Everyday Habits
Your testicles are remarkably sensitive to temperature. An increase of just a few degrees can stall sperm production, and the effects of heat don’t show up right away. Sperm take about 64 days to fully develop, so damage from heat typically appears in semen analysis results four to seven weeks later.
The good news is that heat-related drops are often reversible. In one study, men who had been using hot tubs or taking hot baths for at least 30 minutes per week stopped the practice. Three months later, nearly half of them saw their motile sperm counts jump by an average of 491%. Recovery after heat exposure generally takes 10 to 12 weeks.
Common sources of excess scrotal heat include:
- Hot tubs and saunas. One study found that roughly two and a half hours of sauna exposure every two weeks cut sperm counts by up to 50%.
- Prolonged sitting. Sitting with your thighs pressed together for 60 minutes raised scrotal temperature by 2.1°C in one study. Adding a laptop on your lap increased it by another half a degree.
- Tight underwear. Depending on the style and your posture, tight-fitting underwear can raise testicular temperature by 0.5 to 0.8°C. That sounds small, but the margin for normal sperm production is narrow.
Body Weight
Carrying excess weight affects sperm production through multiple pathways. Fat tissue converts testosterone into estrogen, shifting the hormonal balance away from what the testicles need. A meta-analysis in Frontiers in Endocrinology found that normal-weight men had significantly higher sperm concentrations, total sperm counts, and sperm motility than obese men. Every five-unit increase in BMI was associated with roughly a 2.4% reduction in total sperm count. That percentage compounds quickly for men who are well into the obese range.
Medications and Substances
Several common medications can lower sperm count, sometimes without you realizing it.
- Testosterone therapy. Paradoxically, supplementing testosterone is one of the most reliable ways to tank your sperm count. External testosterone tells the brain to stop sending the signals that drive sperm production.
- Finasteride. Used for hair loss and enlarged prostate, finasteride can reduce ejaculate volume, sperm count, and sperm motility.
- Antidepressants. SSRIs and tricyclic antidepressants have been linked to lower sperm counts along with sexual side effects like delayed orgasm and reduced desire.
- Opioid painkillers. Even at low doses, opioids can decrease sperm motility. Higher doses can suppress testosterone and interfere with sperm production more broadly.
- Chemotherapy. Many chemotherapy drugs damage sperm-producing cells directly. Whether production recovers depends on the specific drug, dose, and duration of treatment.
- Anabolic steroids. These act like external testosterone, shutting down the body’s own production. The effect can persist for months after stopping.
Recreational substances matter too. Heavy alcohol use, regular marijuana use, and tobacco are all associated with decreased fertility in men.
Infections
Sexually transmitted infections, particularly gonorrhea and chlamydia, can cause lasting damage when left untreated. These bacteria can trigger inflammation of the epididymis, the coiled tube behind each testicle where sperm are stored and mature. If scarring develops, it can physically block sperm from being released during ejaculation. Gonorrhea can also scar the urethra and disrupt ejaculation itself.
Mumps orchitis (a viral infection of the testicle, usually from mumps contracted after puberty) is another well-known cause of reduced sperm production. The inflammation can damage the sperm-producing tissue permanently, though the severity varies.
Pesticide and Chemical Exposure
Occupational exposure to certain chemicals is a less obvious but documented cause. A meta-analysis of studies on organophosphate pesticides, widely used in agriculture, found significantly lower sperm counts, sperm concentration, and motility in exposed workers compared to unexposed controls. The effects were consistent enough across studies to show a clear pattern, even though hormone levels like testosterone, FSH, and LH weren’t significantly altered. This suggests the chemicals may damage sperm-producing cells directly rather than disrupting hormonal signaling.
Other environmental exposures linked to lower sperm counts include heavy metals (lead, cadmium) and industrial chemicals like phthalates and bisphenol A (BPA), which are found in some plastics and personal care products.
What Recovery Looks Like
A full cycle of sperm production takes about 64 days, with four 16-day stages from early stem cell to mature sperm. This means any change you make today, whether stopping a medication, losing weight, or avoiding heat, won’t show up in a semen analysis for roughly two to three months. That timeline applies equally to improvements and setbacks.
For lifestyle-related causes, the outlook is generally encouraging. Heat damage is often fully reversible within a few months. Weight loss can shift hormone levels back toward a fertile balance. Stopping a problematic medication (with your doctor’s guidance) allows the hormonal chain to restart. For structural problems like varicocele, surgical repair improves sperm parameters in many men, though results take several months to appear.
When couples are trying to conceive, fertility guidelines recommend that both partners be evaluated at the same time rather than assuming the issue is on one side. A semen analysis paired with a reproductive history is the standard starting point for the male evaluation. If you’ve gone through assisted reproduction cycles that haven’t worked, or your partner has experienced recurrent pregnancy losses, a more thorough male workup is worth pursuing even if an earlier semen analysis looked normal.