Finasteride is a medication prescribed to address conditions linked to the male hormone dihydrotestosterone (DHT). It is commonly used in a 1-milligram dose for male pattern hair loss (androgenetic alopecia) and in a 5-milligram dose for an enlarged prostate (benign prostatic hyperplasia, or BPH). The drug functions as a specific inhibitor of the Type II 5-alpha reductase enzyme, which converts testosterone into the more potent DHT. Since DHT drives both hair follicle miniaturization and prostate growth, blocking this conversion effectively lowers DHT concentrations in the blood and tissue.
Average Half-Life and Elimination Time
The time it takes for finasteride molecules to physically exit the body is measured by the drug’s elimination half-life—the time required for the concentration of the drug in the plasma to be reduced by half. For a healthy young adult, the mean half-life is approximately six to eight hours, regardless of the 1-milligram or 5-milligram dose.
The body eliminates finasteride through metabolism in the liver, primarily via the cytochrome P450 3A4 (CYP3A4) enzyme system. The drug is broken down into various inactive metabolites. These metabolites are then excreted from the body through two main routes: 32 to 46 percent is eliminated through the urine, and the majority (51 to 64 percent) is eliminated through the feces.
For a substance to be virtually eliminated from the bloodstream, it takes about four to five half-lives. Given finasteride’s mean half-life of six to eight hours, the drug is cleared from the system within 48 to 72 hours for most healthy individuals. Within two to three days after the last dose, the parent finasteride molecules are no longer present at a significant concentration in the blood.
How Age and Health Influence Clearance
While clearance is rapid for most people, physiological factors influence the rate at which finasteride is processed. Age is one factor, as the clearance rate decreases slightly in older patients. In men over 70, the elimination half-life is often prolonged to about eight hours, compared to the six-hour average for younger adults.
This delay is attributed to the natural decline in hepatic (liver) function that occurs with age. However, this change is not significant enough to warrant a dosage adjustment for elderly patients. Since the liver is the primary site of metabolism, caution is advised for individuals with known liver function abnormalities, as impairment is expected to slow the drug’s clearance rate.
Patients with kidney impairment do not require dosage modification. Finasteride is primarily eliminated through conversion to inactive metabolites and excretion via the feces, which acts as a compensatory pathway. If the kidneys are not functioning optimally, the body increases the amount of metabolites excreted through the digestive system, ensuring efficient overall clearance.
The Timeline for DHT Recovery and Effect Reversal
The physical presence of the drug in the bloodstream is distinct from the duration of its biological effects. While finasteride molecules are eliminated within a few days, the effect persists much longer. Finasteride works by binding stably to the 5-alpha reductase enzyme. DHT suppression continues until the body synthesizes and replaces the inhibited enzyme.
After discontinuing the medication, serum DHT levels begin to rise as new enzyme is produced. Clinical studies indicate that serum DHT levels return to their pre-treatment baseline within about 14 days of the last dose.
The reversal of the therapeutic effect, such as protection against hair loss, takes considerably longer than the hormonal rebound. Since the hair growth cycle is slow, the return to the pre-treatment hair loss rate is a gradual process. Individuals may begin to notice increased hair shedding within three to six weeks of stopping the medication.
The full reversal of hair loss benefits, where the hair returns to its pre-finasteride state, can take four to six months up to a full year. If the medication was taken for BPH, the prostate volume and associated symptoms can also take several months to return to baseline levels. If the drug was stopped due to side effects, most symptoms resolve within a few weeks to a month as the body’s hormonal balance readjusts.