Vyvanse and Finasteride Interaction: What Patients and Clinicians Need to Know

What the labels say

The Propecia (finasteride 1 mg) label's Drug Interactions section states that no drug interactions of clinical importance have been identified and that finasteride does not appear to affect the cytochrome P450-linked drug-metabolizing enzyme system [1]. Antipyrine, digoxin, propranolol, theophylline and warfarin were tested in humans with no clinically meaningful interactions [1].

The 5 mg tablet is a separate product, Proscar, with its own label for benign prostatic hyperplasia [4][5].

The Vyvanse label takes a different approach: a table of drugs with clinically important interactions. It lists MAO inhibitors, contraindicated during and within 14 days of stopping one because of hypertensive crisis risk; serotonergic drugs, because of serotonin syndrome; CYP2D6 inhibitors, which may raise exposure to dextroamphetamine; urinary alkalinizing agents, which the label says should be avoided with Vyvanse; urinary acidifying agents, which lower amphetamine levels; and tricyclic antidepressants [2]. Finasteride does not appear in that table [2].

Why the enzyme story is short

Lisdexamfetamine is an inactive prodrug of dextroamphetamine. It is converted to dextroamphetamine and l-lysine mainly in blood by red blood cells, not by cytochrome P450 enzymes [2]. Finasteride, by contrast, is extensively metabolized in the liver mainly by CYP3A4 [1]. The finasteride label states that the drug does not appear to affect the cytochrome P450 system [1], and the only enzyme-related entry in the Vyvanse interaction table concerns CYP2D6 inhibitors [2].

Side effects the labels list

In the 4-week adult ADHD trial, Vyvanse produced decreased appetite in 27% versus 2% on placebo, insomnia in 27% versus 8%, and dry mouth in 26% versus 3% [2]. In the two 12-week binge eating disorder trials, dry mouth was 36% versus 7% and insomnia 20% versus 8% [2]. CNS stimulants raise blood pressure by a mean of about 2 to 4 mm Hg and heart rate by about 3 to 6 bpm, with larger rises in some patients [2]. The label also warns that stimulants may trigger a manic or mixed episode in bipolar disorder and can cause new psychotic or manic symptoms in about 0.1% of treated patients versus 0% on placebo in pooled short-term trials [2].

Finasteride's three 12-month controlled trials in men aged 18 to 41 (945 on finasteride, 934 on placebo) found decreased libido in 1.8% versus 1.3%, erectile dysfunction in 1.3% versus 0.7%, and ejaculation disorder in 1.2% versus 0.7%; 3.8% of finasteride users reported at least one of these versus 2.1% on placebo (p=0.04) [1]. The postmarketing section lists depression and suicidal ideation and behavior, along with sexual dysfunction that continued after stopping treatment [1]. Those reports are voluntary, come from a population of uncertain size, and a causal relationship cannot reliably be established [1].

The labels describe insomnia with Vyvanse and postmarketing reports of depression with finasteride, but they do not show that taking the drugs together increases either effect [1][3][2].

Practical details from the labels

Vyvanse is taken in the morning with or without food, and the label says to avoid afternoon doses because of insomnia [2]. The label says capsules can be swallowed whole, or opened and mixed with yogurt, water or orange juice and consumed immediately [2].

The Propecia label says it is indicated for male pattern hair loss in men only, is contraindicated during pregnancy, and warns of fetal risk from handling crushed or broken tablets during pregnancy [1]. The label gives the dose as one 1 mg tablet once daily with or without meals; it says daily use for three months or more is generally necessary before benefit is observed, and that withdrawal of treatment leads to reversal of effect within 12 months [1].

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Is a dose change needed?

Neither label describes a dose adjustment when the two drugs are used together [1][2]. The Vyvanse label starts ADHD treatment at 30 mg every morning, adjusted by 10 or 20 mg at about weekly intervals up to a maximum of 70 mg per day [2]. Propecia is a fixed 1 mg once daily [1].