Is 20 mg of tadalafil the same as 100 mg of sildenafil?

No label, guideline or trial says so. The claim comes from reading the two labelled ladders across each other — sildenafil is 25, 50 and 100 mg and tadalafil is 5, 10 and 20 mg, exactly 0.2× at every rung — which is three companies choosing round numbers, not a measurement of potency. The same reading makes tadalafil and vardenafil 1 : 1 despite terminal half-lives of 17.5 hours and 4 to 5 hours, which nobody believes, and it makes avanafil the weakest of the four by a factor of ten, which is why no circulating equivalence table includes avanafil. If you are switching, the new drug's own starting dose is the only number a label will give you.

Does tadalafil really last 36 hours?

The label says: "CIALIS for use as needed was shown to improve erectile function compared to placebo up to 36 hours following dosing." That is an efficacy window — how long after a single dose the drug still beat placebo in a trial — not a promise about any individual evening and certainly not a 36-hour erection. Sildenafil's label documents its effect to 4 hours and adds that "the response was diminished compared to 2 hours", so both windows are front-loaded. What the 36 hours does buy, reliably, is the ability to stop planning.

Which one works faster?

Neither label states an onset of action, and this is a place the two labels are not comparable. Sildenafil's dosage section instructs "approximately 1 hour before sexual activity" and permits anywhere from 30 minutes to 4 hours before. Tadalafil's dosage section gives no time at all — only "prior to anticipated sexual activity" — so the nearest figure is its median time to peak, 2 hours, with a range of 30 minutes to 6. Those are two different kinds of number and putting them in the same column without saying so is how "tadalafil takes twice as long" gets repeated. On plasma kinetics, sildenafil peaks sooner; on what the labels instruct, only one of them instructs anything.

Can I take both, or switch between them?

Taking two PDE5 inhibitors together has not been studied and is not recommended on the labels, so it is not something to do on your own. Switching is ordinary and the labels are silent on it — neither one mentions prior PDE5 use, a converted dose or a switching schedule anywhere. In practice that means starting the new drug at its own labelled starting dose. The AUA guideline supports exactly this: "As part of the process of identifying the optimal dose, men may be offered dosing frequency changes or different PDE5i."

Is the long half-life a problem?

It has one concrete cost, stated on the label. If a nitrate is ever medically necessary in a life-threatening situation, tadalafil's label requires that "at least 48 hours should elapse after the last dose of CIALIS before nitrate administration is considered." Sildenafil's label gives no interval at all, saying instead that "it is unknown when nitrates, if necessary, can be safely administered" — which is an absence of information rather than a shorter window. All four labels contraindicate concomitant use with organic nitrates or nitric oxide donors in any form — nitroglycerin, isosorbide mononitrate or dinitrate, and amyl or butyl nitrite "poppers". A PDE5 inhibitor taken with a nitrate can drop blood pressure to a dangerous level. Guanylate cyclase stimulators such as riociguat are contraindicated on all four labels as well.

As generics, both are pennies a dose at what pharmacies pay: in the CMS NADAC file dated 2026-09-16, $0.10577 for a 50 mg sildenafil tablet and $0.15748 for a 10 mg tadalafil. The interesting figure is the daily regimen: tadalafil 5 mg every day comes to $3.11 a month at those rates, which is what roughly 29 as-needed sildenafil doses would cost — and no label allows more than 31 doses in a month. The branded products are a different world: Viagra 100 mg is 655 times its own generic. None of these is a retail price; NADAC records what pharmacies paid on invoice.

Head to head

Sildenafil vs Tadalafil

Viagra against Cialis, from both FDA labels. One difference is enormous and fully documented. The one everybody asks about — how many milligrams equal how many — is not on either label.

Reviewed September 2026

The short answer

The difference people have heard about is real and it is on both labels. Sildenafil's label documents an effect "for up to 4 hours"; tadalafil's documents improved erectile function against placebo "up to 36 hours following dosing". That is nine times the window, stated the same way by both manufacturers, and it is the one thing that genuinely separates these two drugs.

Two smaller differences decide it for a lot of men. Tadalafil's absorption is "not influenced by food" while a high-fat meal delays sildenafil's peak by a full hour and cuts it by 29% — so one of them has to be planned around dinner and the other does not. And only tadalafil has a licensed once-daily regimen, 2.5 mg rising to 5 mg, which takes the timing question off the table entirely.

The question this page will not answer is how many milligrams of one equal how many of the other, because no label, guideline or trial says. No FDA label for any of the four PDE5 inhibitors states a dose equivalence with any other, and no published trial establishes one. Twenty milligrams of tadalafil is not "equal to" 100 mg of sildenafil in any labelled or trial sense. The only dose a label will give you for a drug is that drug's own.

The long half-life is not free. Tadalafil's label says at least 48 hours must elapse after a dose before a nitrate can even be considered in a life-threatening emergency; sildenafil's says only that "it is unknown when nitrates, if necessary, can be safely administered". Neither is a safe combination, but 36 hours of cover is also 48 hours of exposure.

4 of 16 dimensions are materially the same. They are marked below rather than dressed up as differences.

Sildenafil

Viagra

The original PDE5 inhibitor. Taken about an hour before, with a documented effect to four hours and a real food effect.

Dosing protocol →

Tadalafil

Cialis

The long one. Effect documented to 36 hours, no food effect at all, and the only one of the four with a licensed once-daily regimen.

Dosing protocol →

Side by side

Every row carries how well supported it is. A claim that only reflects common practice is labelled as such rather than presented as fact.

Dimension	Sildenafil	Tadalafil	Basis
Duration of effect	up to 4 hours	up to 36 hours	labelBoth labels state it in the same form. Tadalafil, verbatim: "CIALIS for use as needed was shown to improve erectile function compared to placebo up to 36 hours following dosing." That is an efficacy window against placebo, not a 36-hour erection. Sildenafil's label adds that at 4 hours "the response was diminished compared to 2 hours".
Dose equivalence between the twosame	Not stated on the label	Not stated on the label	labelNeither label mentions the other drug, a conversion, or switching. The circulating 5:1 ratio is the two labelled ladders read across — 25, 50 and 100 mg against 5, 10 and 20 mg, 0.2× at every rung — which is round numbers, not a measurement. The same method makes tadalafil and vardenafil 1 : 1 despite half-lives of 17.5 hours and 4 to 5.
When to take it	approximately 1 hour before	median 2 hr to peak	labelNot the same kind of figure, which is why the basis is printed with each. Sildenafil's is a dosage-section instruction and its label permits 30 minutes to 4 hours before. Tadalafil's label gives no lead time at all — only "prior to anticipated sexual activity" — so the figure shown is median Tmax. "Tadalafil works in 30 minutes" appears nowhere on its label.
Food effect	A high-fat meal pushes the peak back a full hour and cuts it by 29% — the largest food effect of the four.	None. The label states absorption is not influenced by food at all.	labelTadalafil, verbatim: "The rate and extent of absorption of tadalafil are not influenced by food; thus CIALIS may be taken with or without food." Sildenafil's label states a mean Tmax delay of 60 minutes and a mean Cmax reduction of 29% with a high-fat meal. One of these has to be planned around dinner.
Terminal half-life	about 4 hours	17.5 hours	labelFour hours against seventeen and a half. This is the number behind the duration, the nitrate interval and the once-daily regimen all at once.
Labelled as-needed range	25–100 mg	5–20 mg	labelEach label's own ladder: start at 50 mg for sildenafil and 10 mg for tadalafil, then titrate within that range. Neither range is expressed in terms of the other, and a dose is only meaningful for the drug whose label states it.
Licensed once-daily regimen	None on the label	2.5 mg daily, may increase to 5 mg	labelTadalafil, verbatim: "The recommended starting dose of CIALIS for once daily use is 2.5 mg, taken at approximately the same time every day, without regard to timing of sexual activity." There is no once-daily sildenafil regimen on any US label, and taking one daily is outside what its label describes.
Benign prostatic hyperplasia indication	No	Yes — 5 mg once daily	labelTadalafil's label carries a separate BPH indication at the same daily dose used for ED, which is why a man with both conditions is often given tadalafil. The same section warns it "is not recommended for use in combination with alpha-blockers for the treatment of BPH."
Maximum dosing frequencysame	Once per day	Once per day	labelBoth labels cap as-needed use at once per day. Tadalafil's adds "in most patients".
Nitratessame	Absolute contraindication	Absolute contraindication	labelNot a caution and not a "discuss with your doctor". Both labels contraindicate concomitant organic nitrates or nitric oxide donors in any form, which includes amyl and butyl nitrite "poppers". Guanylate cyclase stimulators such as riociguat are contraindicated on both as well.
Interval before a nitrate in an emergency	Label states none — "it is unknown"	At least 48 hours	labelThe cost of the long half-life, stated by the label itself: tadalafil requires that "at least 48 hours should elapse after the last dose of CIALIS before nitrate administration is considered." Sildenafil's label offers no interval: "After patients have taken VIAGRA, it is unknown when nitrates, if necessary, can be safely administered." A shorter unknown is not a shorter known.
Starting dose on an alpha-blocker	Start at 25 mg; be stable on the alpha-blocker first	Start at the lowest recommended dose — no figure given	labelSildenafil’s dosage section names a number and tadalafil’s does not — it says only that CIALIS "should be initiated at the lowest recommended dose". Both require the man to be stable on the alpha-blocker first. For BPH, tadalafil "is not recommended for use in combination with alpha-blockers".
Head-to-head trial resultsame	No difference found, at either 50 mg or 100 mg	No difference found, at 20 mg	literatureENDOTRIAL (PMID 19570039), four arms including both drugs: "An overall equivalence was demonstrated in the subjective perception of treatment benefits for all the PDE5i tested." It yields no ratio — 2 different sildenafil doses both matched tadalafil 20 mg.
How much dose itself matters	Doubling 50 mg to 100 mg moved the average IIEF-EF score 1.2 points	Not separately quantified by the guideline	guidelineThe AUA found "the differences in response rates between dose groups were extremely small, rarely statistically significant, and generally not clinically significant". If doubling within one drug buys a point on a 30-point scale, a milligram mapping between drugs matters less than it looks.
What a pharmacy paid, per dose	$0.10577 for 50 mg	$0.15748 for 10 mg	literatureGeneric, CMS NADAC file dated 2026-09-16 — invoice cost to a retail pharmacy, not a price you can pay. Both are pennies. The branded products are not: Viagra 100 mg is 655 times its generic and Cialis 10 mg is 341 times its own.
Cost of daily tadalafil against as-needed	Not applicable — no daily regimen	$3.11 a month at 5 mg daily	literatureAt those acquisition rates, a month of daily tadalafil costs about what 29 as-needed sildenafil 50 mg doses cost — and no label permits more than 31 doses in a month. On acquisition cost alone, daily dosing is close to a free option. Whether it suits you is a clinical question with two separate labelled regimens behind it.

label: Stated on the FDA label
guideline: Clinical practice guideline
literature: Published literature
practice: Common practice, not documented

Which one

Choose Sildenafil if

Sex is planned and you would rather the drug be gone the next morning. Four hours in, four hours out.
You take a nitrate only in the sense that it might one day be needed in an emergency, and a 48-hour exclusion window worries you more than a 4-hour one.
You get tadalafil-specific side effects — back pain and muscle ache are the ones its label reports and sildenafil's does not.
You are happy to keep it away from a heavy meal and take it an hour ahead.

Choose Tadalafil if

You want the timing question to stop existing. Thirty-six hours of labelled cover, or a daily tablet and no planning at all.
You eat before sex. Tadalafil's label says absorption is "not influenced by food"; sildenafil's says a high-fat meal costs you an hour and 29% of the peak.
You also have benign prostatic hyperplasia — tadalafil carries a BPH indication at the same daily dose, and sildenafil carries none.
Spontaneity is the point. This is the only one of the two with a licensed regimen that does not require you to decide in advance.

Neither is stronger than the other, and nobody can tell you how many milligrams of one equal how many of the other because no label, guideline or trial states it. The honest way to switch is to start the new drug at its own labelled starting dose and titrate on how it works for you — which is exactly what the AUA guideline recommends, and it recommends it because the evidence for picking a dose any other way does not exist. The published claims that do circulate disagree with each other: two sources put vardenafil at 5× and 10× sildenafil, a factor of 2 apart, and neither cites anything.

Frequently asked questions

Is 20 mg of tadalafil the same as 100 mg of sildenafil?: No label, guideline or trial says so. The claim comes from reading the two labelled ladders across each other — sildenafil is 25, 50 and 100 mg and tadalafil is 5, 10 and 20 mg, exactly 0.2× at every rung — which is three companies choosing round numbers, not a measurement of potency. The same reading makes tadalafil and vardenafil 1 : 1 despite terminal half-lives of 17.5 hours and 4 to 5 hours, which nobody believes, and it makes avanafil the weakest of the four by a factor of ten, which is why no circulating equivalence table includes avanafil. If you are switching, the new drug's own starting dose is the only number a label will give you.
Does tadalafil really last 36 hours?: The label says: "CIALIS for use as needed was shown to improve erectile function compared to placebo up to 36 hours following dosing." That is an efficacy window — how long after a single dose the drug still beat placebo in a trial — not a promise about any individual evening and certainly not a 36-hour erection. Sildenafil's label documents its effect to 4 hours and adds that "the response was diminished compared to 2 hours", so both windows are front-loaded. What the 36 hours does buy, reliably, is the ability to stop planning.
Which one works faster?: Neither label states an onset of action, and this is a place the two labels are not comparable. Sildenafil's dosage section instructs "approximately 1 hour before sexual activity" and permits anywhere from 30 minutes to 4 hours before. Tadalafil's dosage section gives no time at all — only "prior to anticipated sexual activity" — so the nearest figure is its median time to peak, 2 hours, with a range of 30 minutes to 6. Those are two different kinds of number and putting them in the same column without saying so is how "tadalafil takes twice as long" gets repeated. On plasma kinetics, sildenafil peaks sooner; on what the labels instruct, only one of them instructs anything.
Can I take both, or switch between them?: Taking two PDE5 inhibitors together has not been studied and is not recommended on the labels, so it is not something to do on your own. Switching is ordinary and the labels are silent on it — neither one mentions prior PDE5 use, a converted dose or a switching schedule anywhere. In practice that means starting the new drug at its own labelled starting dose. The AUA guideline supports exactly this: "As part of the process of identifying the optimal dose, men may be offered dosing frequency changes or different PDE5i."
Is the long half-life a problem?: It has one concrete cost, stated on the label. If a nitrate is ever medically necessary in a life-threatening situation, tadalafil's label requires that "at least 48 hours should elapse after the last dose of CIALIS before nitrate administration is considered." Sildenafil's label gives no interval at all, saying instead that "it is unknown when nitrates, if necessary, can be safely administered" — which is an absence of information rather than a shorter window. All four labels contraindicate concomitant use with organic nitrates or nitric oxide donors in any form — nitroglycerin, isosorbide mononitrate or dinitrate, and amyl or butyl nitrite "poppers". A PDE5 inhibitor taken with a nitrate can drop blood pressure to a dangerous level. Guanylate cyclase stimulators such as riociguat are contraindicated on all four labels as well.
Which is cheaper?: As generics, both are pennies a dose at what pharmacies pay: in the CMS NADAC file dated 2026-09-16, $0.10577 for a 50 mg sildenafil tablet and $0.15748 for a 10 mg tadalafil. The interesting figure is the daily regimen: tadalafil 5 mg every day comes to $3.11 a month at those rates, which is what roughly 29 as-needed sildenafil doses would cost — and no label allows more than 31 doses in a month. The branded products are a different world: Viagra 100 mg is 655 times its own generic. None of these is a retail price; NADAC records what pharmacies paid on invoice.

Sources

Next steps

Compare online ED providersWhich platforms prescribe which drug, what a consultation costs, and what you actually receive.Erectile dysfunction, start to finishWhat causes it, what gets ruled out first, and what the treatment ladder looks like.

Other comparisons

Educational reference, not medical advice. Which medication suits you is a decision for your prescriber.