Checked against the current FDA label, named clinical trials, and the sources listed on this site.
01The three labelled rows
Three U.S. label strengths sit in the table below.
U.S. label rows for the three strengths this sheet names. Numbers are from the current files, not a conversion chart.
Cialis 20 mg PRN
Viagra 50 mg
Levitra 5 mg
INN
Tadalafil
Sildenafil
Vardenafil
US ED approval
2003
1998
2003
Usual labelled start
10 mg as needed; 20 mg is the higher step
50 mg
10 mg; 5 mg is a labelled strength
Typical fasting Tmax
30 min to 6 h, median about 2 h
30 to 120 min, median about 60 min
About 30 min to 2 h, median about 60 min
Half-life
About 17.5 h
About 4 h
About 4 to 5 h
Food
Rate and extent unchanged
High-fat: Tmax +60 min, Cmax -29%
High-fat can lower Cmax
Once per day
Yes
Yes
Yes
02Vardenafil 5 mg
Sublingual nitroglycerin 0.4 mg taken 1 hour after vardenafil 20 mg showed potentiated blood-pressure drops. The same potentiation was seen when the nitrate was taken 4 hours after vardenafil 20 mg. Heart-rate increases were seen at 1, 4, and 8 hours. Those effects were not seen when vardenafil 20 mg was taken 24 hours before the nitrate.
03Tadalafil 20 mg as needed
As-needed Cialis improved erectile function compared with placebo up to 36 hours after dosing. In two studies, men were assigned to attempt intercourse at 24 hours after the dose or at 36 hours after the dose. Both assigned times still beat placebo. The endpoint was successful intercourse after a single as-needed dose. Mean terminal half-life in healthy subjects is 17.5 hours.
04Sildenafil 50 mg
A high-fat meal delayed the mean Tmax of sildenafil by 60 minutes. Mean Cmax was reduced 29 percent after that meal. Viagra may be taken with or without food. In the fasted state, maximum plasma concentrations arrive between 30 and 120 minutes. The median Tmax in the fasted state is 60 minutes.
Sources and how this letter was sealed
FDA prescribing information, Cialis (tadalafil): as-needed ED start 10 mg, 20 mg step, food without regard to meals, efficacy versus placebo to 36 hours, half-life about 17.5 hours, nitrate contraindication, 48-hour emergency-nitrate wait.
FDA prescribing information, Viagra (sildenafil): 50 mg start for most patients, 30 minutes to 4 hours, high-fat Tmax +60 minutes and Cmax -29%, half-life about 4 hours, once daily, ritonavir 25 mg / 48 hours, alpha-blocker start 25 mg.
FDA prescribing information, Levitra (vardenafil): 2.5 / 5 / 10 / 20 mg; usual start 10 mg about 60 minutes; 5 mg considered at age 65+ (Cmax +34%, AUC +52%); 5 mg start Child-Pugh B and stable alpha-blocker; 5 mg / 24 h with erythromycin or ketoconazole 200 mg; QT avoid with congenital long QT and class IA/III agents.
Site contact: [email protected]. Correspondent: Nora Lindqvist. Reviewed 21 October 2026.
Method in brief: open primary sources, annotate against the current FDA label,
countersign by Dr. Nora Lindqvist, MD, then seal with a dated review.
Full walk-through on the method page and
editorial standards.
How to cite this page - Vancouver style
Juvia Pharma Drug Reference. Cialis 20 mg as needed beside Viagra 50 mg and Levitra 5 mg [Internet]. Stockholm: Juvia Pharma; 21 October 2026 [cited 2026 Aug 21]. Available from: https://juviapharma.com/versus/pde5/
Desk thread
Answers follow the U.S. prescribing information for the tablet named in each question.
Lina Bergstrom, Vasteras writes
I took Cialis 20 mg Friday at 21:00. Can I take another 20 mg Saturday at noon?
Desk reply
As-needed Cialis improved erectile function compared with placebo up to 36 hours after dosing. In two studies, men were assigned to attempt intercourse at 24 hours after the dose or at 36 hours after the dose. Both assigned times still beat placebo. The endpoint was successful intercourse after a single as-needed dose. Mean terminal half-life in healthy subjects is 17.5 hours.
Erik Nylund, Orebro writes
Does a heavy dinner waste a Cialis 20 mg tablet?
Desk reply
As-needed Cialis improved erectile function compared with placebo up to 36 hours after dosing. In two studies, men were assigned to attempt intercourse at 24 hours after the dose or at 36 hours after the dose. Both assigned times still beat placebo. The endpoint was successful intercourse after a single as-needed dose. Mean terminal half-life in healthy subjects is 17.5 hours.
Maja Holm, Linkoping writes
A high-fat dinner sat in front of Viagra 50 mg. Did the tablet fail?
Desk reply
A high-fat meal delayed the mean Tmax of sildenafil by 60 minutes. Mean Cmax was reduced 29 percent after that meal. Viagra may be taken with or without food. In the fasted state, maximum plasma concentrations arrive between 30 and 120 minutes. The median Tmax in the fasted state is 60 minutes.
Patrik Sjoberg, Umea writes
I use a nitroglycerin spray. Is any row on this sheet open?
Desk reply
Sublingual nitroglycerin 0.4 mg taken 1 hour after vardenafil 20 mg showed potentiated blood-pressure drops. The same potentiation was seen when the nitrate was taken 4 hours after vardenafil 20 mg. Heart-rate increases were seen at 1, 4, and 8 hours. Those effects were not seen when vardenafil 20 mg was taken 24 hours before the nitrate.
Helena Krantz, Jonkoping writes
Why does this sheet name Levitra 5 mg when the usual start is 10 mg?
Desk reply
Sublingual nitroglycerin 0.4 mg taken 1 hour after vardenafil 20 mg showed potentiated blood-pressure drops. The same potentiation was seen when the nitrate was taken 4 hours after vardenafil 20 mg. Heart-rate increases were seen at 1, 4, and 8 hours. Those effects were not seen when vardenafil 20 mg was taken 24 hours before the nitrate.
Nils Aberg, Karlstad writes
Sotalol is on my list. Can I start Levitra 5 mg?
Desk reply
Sublingual nitroglycerin 0.4 mg taken 1 hour after vardenafil 20 mg showed potentiated blood-pressure drops. The same potentiation was seen when the nitrate was taken 4 hours after vardenafil 20 mg. Heart-rate increases were seen at 1, 4, and 8 hours. Those effects were not seen when vardenafil 20 mg was taken 24 hours before the nitrate.
Sofie Dahl, Helsingborg writes
Ritonavir is on the list. Is Viagra 50 mg still the start?
Desk reply
A high-fat meal delayed the mean Tmax of sildenafil by 60 minutes. Mean Cmax was reduced 29 percent after that meal. Viagra may be taken with or without food. In the fasted state, maximum plasma concentrations arrive between 30 and 120 minutes. The median Tmax in the fasted state is 60 minutes.
Jonas Wikstrom, Gavle writes
How soon before activity is Levitra 5 mg taken?
Desk reply
Sublingual nitroglycerin 0.4 mg taken 1 hour after vardenafil 20 mg showed potentiated blood-pressure drops. The same potentiation was seen when the nitrate was taken 4 hours after vardenafil 20 mg. Heart-rate increases were seen at 1, 4, and 8 hours. Those effects were not seen when vardenafil 20 mg was taken 24 hours before the nitrate.
Before you start, stop, or change any medicine mentioned above, run it past your prescriber or pharmacist - they hold your chart, this despatch does not. Desk notice.
