The 5ARIs, finasteride and dutasteride, are used to treat benign prostate hyperplasia and lower urinary tract symptoms. At much lower doses, 5ARI treatment reduces male hair loss. These drugs inhibit the conversion of testosterone to the more active dihydrotestosterone (DHT). In men taking these medications, DHT levels are reduced by some 90% while testosterone levels remain relatively stable. Well known for their negative effects on libido and erectile function, 5ARIs also cause ejaculatory dysfunction in some men, having the potential to decrease semen quality. In fact, some studies of men treated with these drugs have reported lower total sperm count, along with lower sperm motility, although the changes are probably insufficient to reduce fertility in men with normal semen before treatment. There is a population of men with more severely decreased sperm numbers; as low as 10% of pretreatment values. Fewer studies have looked at the lower doses used for male alopecia, indicating little affect in men with normal semen quality, but a negative effect on sperm numbers in men with oligozoospermia. There have been no studies looking at fertility endpoints for these medications.