Finasteride and Caffeine Interaction: What the Evidence Actually Shows

Is there a finasteride-caffeine interaction?

The finasteride label's drug interactions section is short and specific. It 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]. It names the compounds that were actually tested in people: antipyrine, digoxin, propranolol, theophylline and warfarin, with no clinically meaningful interactions found [1]. Caffeine is not among them.

Theophylline, a related methylxanthine, was tested with finasteride and produced no clinically meaningful interaction [1]. That is a theophylline result, not a caffeine one, and the label lists no study of finasteride given with caffeine.

Where each drug is cleared

Finasteride is a competitive, specific inhibitor of Type II 5-alpha-reductase, the enzyme that converts testosterone to dihydrotestosterone (DHT) [1]. A 1 mg tablet suppresses serum DHT by about 65% within 24 hours [1]. The tablet is about 65% bioavailable, absorption is not affected by food, peak levels arrive 1 to 2 hours after a dose, and the drug is extensively metabolized in the liver mainly by CYP3A4. Mean terminal half-life is about 5 to 6 hours in men aged 18 to 60 and about 8 hours in men over 70; roughly 39% is excreted in urine as metabolites and 57% in feces [1].

Caffeine takes a different route. CYP1A2 carries out 3-N-demethylation to paraxanthine, the main pathway in humans, and also contributes to the other demethylations; under 2% of a dose is excreted unchanged in urine [2][6]. CYP3A4 does contribute to caffeine 8-hydroxylation, but that reaction is not specific enough to be used as a marker of CYP3A4 activity [2]. Caffeine is the FDA's clinical index substrate for CYP1A2, listed alongside tizanidine [7].

What actually changes caffeine levels

Some medicines and hormonal states can change how quickly caffeine is cleared; examples include fluvoxamine, ciprofloxacin, estrogen-containing oral contraceptives and pregnancy [8][10][11][3].

Fluvoxamine is the strongest example of a drug that does. In a crossover study of 8 healthy volunteers given 200 mg caffeine, total caffeine clearance fell from 107 to 21 mL/min and half-life rose from 5 to 31 hours on fluvoxamine; the authors warned that caffeine intoxication is possible [8]. A second study of 7 volunteers given 250 mg caffeine found clearance falling from 105 to 9.1 mL/min and half-life rising from 4.9 to 56 hours, with no measurable change in alertness, psychomotor performance or EEG effects after a single dose [9].

Ciprofloxacin is another. The Cipro label states that ciprofloxacin inhibits CYP1A2 and lists caffeine and xanthine derivatives as affected: reduced clearance, higher levels and a longer half-life, with a recommendation to use caution and monitor for xanthine toxicity [10].

Hormones and smoking matter too. In one study, women who had taken low-dose estrogen oral contraceptives for more than three months had a caffeine half-life of 7.88 hours versus 5.37 hours in 9 matched non-smoking controls, with clearance of 1.05 versus 1.75 mL/min/kg [11]. Smokers cleared caffeine faster than non-smokers, with half-lives of 3.5 versus 6.0 hours in groups of 13 [12]. Pregnancy lengthens it: mean 3.4 hours in 25 men and non-pregnant women versus 8.3 hours in 57 pregnant women [3].

Side effects of each, listed separately

In three 12-month controlled trials of the 1 mg dose (945 men on finasteride, 934 on placebo), decreased libido was reported by 1.8% versus 1.3%, erectile dysfunction by 1.3% versus 0.7%, and ejaculation disorder by 1.2% versus 0.7%; 3.8% of men on finasteride reported one or more of these versus 2.1% on placebo [1]. A meta-analysis of 15 randomized placebo-controlled trials (4,495 men with androgenetic alopecia on finasteride 1 mg or dutasteride 0.5 mg) found a relative risk of sexual dysfunction of 1.66 for finasteride alone (95% CI 1.20 to 2.30) [13]. The Propecia label also lists sexual dysfunction that continued after discontinuation, and depression, suicidal ideation and behavior, as postmarketing reports; it states that these reports are voluntary, come from a population of uncertain size, and that a causal relationship cannot reliably be established [1].

Caffeine has its own documented effects. In a randomized study, 400 mg of caffeine taken at bedtime, 3 hours before bedtime or 6 hours before bedtime each significantly disturbed sleep compared with placebo (P<0.05 for all) [14]. High caffeine intake is associated with higher anxiety levels, particularly in people predisposed to anxiety disorders [15]. The FDA notes wide variation in how sensitive people are to caffeine and how fast they clear it, and that some conditions and medications raise sensitivity [5].

The finasteride label draws no connection between caffeine and finasteride's sexual or psychiatric adverse reactions [1].

How much caffeine is that, in practice?

The FDA cites 400 mg a day as an amount not generally associated with negative effects for most adults, which it equates to about two to three 12-fluid-ounce cups of coffee [5]. Per 12 fl oz, regular brewed coffee runs 113 to 247 mg, an energy drink 41 to 246 mg (54 to 328 mg per 16 fl oz), black tea 71 mg, green tea 37 mg, and a caffeinated soft drink 23 to 83 mg; decaf coffee still carries 2 to 15 mg per 8-fl-oz cup [5]. Caffeine is also added to bars, gum, supplements and some over-the-counter medicines, and the FDA says the body handles natural and added caffeine the same way [5].

The finasteride label gives no caffeine limit. The 400 mg figure is the FDA's general benchmark for healthy adults, not a finasteride-specific threshold [1][5].

Taking finasteride with coffee

Propecia is one 1 mg tablet once daily. The label says it may be taken with or without meals. Daily use for three months or more is usually needed before benefit appears, continued use is needed to sustain it, and stopping leads to reversal of effect within 12 months [1].

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