Testosterone Cypionate and Finasteride Interaction: What Patients and Clinicians Need to Know

What each drug does

Testosterone cypionate is an oil-soluble ester of testosterone given as a deep gluteal intramuscular injection. The Depo-Testosterone label's replacement dose is 50 to 400 mg every two to four weeks, and the intramuscular half-life is about eight days [2]. Exogenous testosterone inhibits pituitary luteinizing hormone and, at large doses, follicle stimulating hormone, which is how it suppresses the body's own testosterone and sperm production [2].

Finasteride is a competitive inhibitor of Type II 5-alpha-reductase with no affinity for the androgen receptor [1]. Type II is the main isozyme in prostate, seminal vesicles, epididymides and hair follicles and accounts for about two-thirds of circulating DHT; finasteride is roughly 100-fold more selective for Type II than for Type I [1]. A 1 mg tablet reaches about 65% suppression of serum DHT within 24 hours, and mean testosterone and estradiol rise by about 15% while staying in the normal range [1]. The enzyme complex turns over slowly, with a half-life of about 30 days for Type II, so the DHT effect outlasts the drug's 5 to 6 hour plasma half-life [1].

Is there a pharmacokinetic interaction?

Finasteride's interaction section states that no drug interactions of clinical importance have been identified, that finasteride does not appear to affect the cytochrome P450 system, and that no clinically meaningful interactions were found with antipyrine, digoxin, propranolol, theophylline or warfarin [1]. The same section lists other medicines used alongside finasteride in clinical studies without evidence of clinically significant interactions; androgens are not on that list [1].

The Depo-Testosterone label says androgens may increase sensitivity to oral anticoagulants, raise serum oxyphenbutazone levels, and decrease blood glucose and insulin requirements in people with diabetes [2].

Finasteride is metabolized mainly by CYP3A4, peaks 1 to 2 hours after a dose, is about 90% protein bound, and has a mean terminal half-life of about 5 to 6 hours in men aged 18 to 60 [1]. Testosterone cypionate's intramuscular half-life is about 8 days [2].

DHT: scalp and prostate

In androgenetic alopecia the balding scalp holds miniaturized follicles and more DHT than hairy scalp, and finasteride lowers both scalp and serum DHT; the label says the relative contribution of those two reductions to the treatment effect has not been defined [1].

Hair efficacy was shown in men with male pattern hair loss taking finasteride alone. Two 1-year placebo-controlled trials in 1553 men aged 18 to 41 taking finasteride 1 mg daily, with 1215 continuing into blinded second-year extensions, found hair counts in a 1-inch circle of vertex scalp (baseline 876 hairs) that were 107 hairs higher than placebo at 1 year and 138 higher at 2 years (P<0.001) [4]. Placebo-treated men kept losing hair [4].

On the prostate side, the finasteride label warns that 5-alpha-reductase inhibitors may increase the risk of high-grade prostate cancer [1]. In the 7-year Prostate Cancer Prevention Trial of 18,882 men aged 55 or older, finasteride 5 mg reduced prostate cancer prevalence from 24.4% to 18.4%, a 24.8% relative reduction, while Gleason 7 to 10 tumors were more common on finasteride (6.4% vs 5.1% of men) [5]. Proscar is not approved to prevent prostate cancer, and the Propecia label says the clinical significance of the high-grade finding for men taking 1 mg is unknown [3][1].

PSA on finasteride

In men aged 18 to 41 taking Propecia, mean PSA fell from 0.7 to 0.5 ng/mL at month 12 [1]. With Proscar 5 mg, PSA falls by about 50% within six months, and that label says to double an isolated PSA value after six months of treatment for comparison with normal ranges [3]. The Propecia label also says any confirmed increase from the lowest PSA value while on finasteride may signal prostate cancer and should be evaluated, even if the value is still within the normal range for men who are not taking a 5-alpha-reductase inhibitor, and that missed doses can affect results [1].

Known or suspected prostate carcinoma is a contraindication to Depo-Testosterone [2].

Sexual function and mood

Both labels list sexual and mood effects.

For finasteride 1 mg, three 12-month controlled trials (945 on drug, 934 on placebo, men 18 to 41) reported drug-related decreased libido 1.8% vs 1.3%, erectile dysfunction 1.3% vs 0.7% and ejaculation disorder 1.2% vs 0.7%; 3.8% vs 2.1% had at least one of these (p=0.04), and each rate fell to 0.3% or lower by year 5 [1]. A meta-analysis of 15 randomized placebo-controlled trials of finasteride or dutasteride in 4,495 men with androgenetic alopecia found a relative risk of sexual dysfunction of 1.66 (95% CI 1.20 to 2.30) in the finasteride subgroup [6].

The Propecia label's postmarketing section lists sexual dysfunction that continued after stopping, including erectile dysfunction, libido disorders, ejaculation disorders and orgasm disorders, along with male infertility and poor seminal quality [1]. Depression and suicidal ideation are also listed as postmarketing reports, with the label noting that these reports are voluntary, come from a population of uncertain size, and cannot establish a causal relationship [7][1]. Neither Propecia nor Proscar has a boxed warning [1][3].

The Depo-Testosterone label lists increased or decreased libido, headache, anxiety and depression among adverse reactions, and gynecomastia that may occasionally persist [2].

Fertility

Exogenous testosterone inhibits pituitary LH and, at large doses, FSH; this feedback can suppress sperm production, and the Depo-Testosterone label lists oligospermia at high dosages [2]. Recovery is possible: in a series of 49 men with azoospermia or severe oligospermia on testosterone, hCG-based combination therapy restored or improved spermatogenesis in 47 (95.9%) at an average of 4.6 months [8], and in a series of 66 men, 70% reached a total motile count above 5 million within 12 months of stopping testosterone and starting hCG plus a selective estrogen receptor modulator [9]. The Endocrine Society guideline recommends against starting testosterone in men planning fertility in the near term [10].

Finasteride's label lists male infertility and poor seminal quality as postmarketing reports, with normalization or improvement reported after stopping [1]. In a 48-week randomized trial of 181 men aged 19 to 41, finasteride 1 mg had no significant effect on sperm concentration, total sperm per ejaculate, motility or morphology, while ejaculate volume fell 0.3 mL vs 0.2 mL on placebo [11]. In a fertility clinic database, 27 of 4,400 men (0.6%) were taking finasteride; after they stopped, sperm counts rose an average of 11.6-fold, and 57% of those with severe oligospermia rose above 15 million/mL [12].

Monitoring the labels call for

The Depo-Testosterone label says to check hemoglobin and hematocrit periodically during long-term use and to monitor blood pressure periodically, and that testosterone products are not recommended in men with uncontrolled hypertension [2]. Injectable testosterone raises hematocrit: across 29 placebo-controlled trials in 3,393 men, intramuscular enanthate or cypionate raised mean hematocrit by 4.0% (95% CI 2.9 to 5.1) versus placebo [13]. FDA's 2025 class-wide labeling action followed ambulatory blood pressure studies that confirmed a blood pressure increase with all testosterone products [14].

For finasteride, the monitoring issue is PSA interpretation. The Propecia label also says women who are pregnant or may become pregnant should not handle crushed or broken tablets, because of the risk to a male fetus; intact coated tablets prevent contact during normal handling [1]. The label's dose is one 1 mg tablet once daily with or without food; it says daily use for three months or more is usually needed before benefit is seen, and that withdrawal leads to reversal of effect within 12 months [1].