Tadalafil vs Sildenafil: What Head-to-Head Trials Show About Timing, Food and Side Effects

Sildenafil (Viagra) and tadalafil (Cialis) are the two most widely used tablets for erectile dysfunction, and they are more alike than the marketing suggests. Both block the same enzyme, both need sexual arousal to work, and when trials have tested them against each other they produce similar erections. The differences that matter are timing, food, a few specific side effects, and the fact that only tadalafil can be taken every day.

This comparison is built from the two US prescribing labels and the largest meta-analysis of trials that tested the drugs head to head, so every number below can be traced to a source listed at the end.

The short answer

If you want a long window and do not want to plan around meals, tadalafil is usually the better fit: its label supports sex up to 36 hours after a dose and food does not change how it is absorbed. If you have sex occasionally, want the drug gone by the next day, or get back pain on tadalafil, sildenafil is the better fit.

In a 2017 meta-analysis of 16 direct comparison trials (PMID 28741090), the two worked about equally well. Men and their partners preferred tadalafil, and it caused less flushing but more back pain and muscle aches.

Tadalafil vs sildenafil at a glance

	Sildenafil (Viagra)	Tadalafil (Cialis)
Usual starting dose	50 mg about 1 hour before sex	10 mg before sex, or 2.5 mg daily
Dose range when taken as needed	25 mg to 100 mg	5 mg to 20 mg
Daily dosing option	No	2.5 mg or 5 mg once a day
Median time to peak blood level	60 minutes (range 30 to 120 minutes)	2 hours (range 30 minutes to 6 hours)
Half-life	About 4 hours	17.5 hours
Label window between dose and sex	30 minutes to 4 hours	Up to 36 hours
Effect of a high-fat meal	Peak level 29% lower, about 1 hour later	None
Other approved uses	Pulmonary arterial hypertension (as Revatio)	Enlarged prostate symptoms (BPH); pulmonary arterial hypertension (as Adcirca)
Nitrates	Contraindicated	Contraindicated; at least 48 hours before an emergency nitrate

How both drugs work

An erection starts when sexual arousal releases nitric oxide in the penis. Nitric oxide raises a messenger molecule called cGMP, which relaxes the smooth muscle in the blood vessels so that blood can flow in. An enzyme called PDE5 breaks cGMP down. Sildenafil and tadalafil both block PDE5, so cGMP lasts longer and an erection is easier to get and to keep.

Two things follow. Neither drug causes an erection without arousal, and neither increases desire. And because the mechanism is identical, choosing between them is about how each molecule behaves in the body, not about which one is stronger.

How fast each one starts working

On an empty stomach, sildenafil reaches its peak blood level at a median of 60 minutes, within a range of 30 to 120 minutes. The label recommends taking it about an hour before sex and allows anywhere from 30 minutes to 4 hours.

Tadalafil peaks later, at a median of 2 hours, with a range of 30 minutes to 6 hours. Many men notice an effect before the peak, but taking tadalafil 30 minutes before sex and judging it on that one attempt is a common reason men decide it does not work. If speed matters most, sildenafil on an empty stomach is the more predictable of the two. For the fastest absorbed drug in the class, see our avanafil guide.

How long each one lasts

This is the biggest practical difference. Sildenafil and its active metabolite both have a half-life of about 4 hours, and its label window closes 4 hours after the dose. Tadalafil has a half-life of 17.5 hours, and its label states that it improved erectile function compared with placebo for up to 36 hours after dosing.

That claim rests on a specific trial reported in the Cialis label. When men attempted intercourse 36 hours after taking 20 mg, 64% of them (88 of 137) reported success, compared with 37% on placebo (49 of 133).

A long window does not mean a 36 hour erection. It means the drug is still active if arousal happens during that time. The flip side is that side effects can last longer too: with tadalafil, back pain and muscle aches typically start 12 to 24 hours after a dose and settle within 48 hours.

Food and alcohol

	Sildenafil	Tadalafil
Taken after a high-fat meal	Peak level 29% lower, reached about 60 minutes later	Rate and extent of absorption not affected
Label advice on food	May be taken with or without food	May be taken without regard to food
Alcohol	50 mg did not add to the blood pressure effect of alcohol at a blood alcohol level of 0.08% in healthy volunteers	5 or more units can increase dizziness, headache and blood pressure drop on standing

In practice, a large or fatty dinner is one of the most common reasons sildenafil seems to fail. If your evenings usually involve a big meal before sex, tadalafil removes that variable. Heavy drinking works against both drugs, because alcohol itself makes erections harder to achieve.

Which works better? What head-to-head trials show

The strongest evidence is the 2017 systematic review and meta-analysis by Gong and colleagues (PMID 28741090), which pooled 16 trials that compared tadalafil directly with sildenafil. On erectile function scores and overall side effect rates the two came out about the same. Tadalafil did better on psychological outcomes such as sexual confidence.

The original registration trials for each drug tell a similar story. They were separate studies in different men, so they show the size of the effect rather than a head-to-head result:

Trial	Drug	Successful intercourse attempts	Placebo
12 week flexible dose study, 329 men (Goldstein 1998, PMID 9580646)	Sildenafil up to 100 mg	69%	22%
11 pooled 12 week trials, 2,102 men (Carson 2004, PMID 15180622)	Tadalafil 20 mg	68%	31%

Both drugs lift success from roughly a quarter to a third of attempts on placebo to about two thirds on treatment. Neither is the stronger drug. The better one for you is the one whose timing and side effects suit your life.

Which one men prefer

Preference is where tadalafil pulls ahead. In the same meta-analysis, nine studies asked men which drug they preferred after trying both, and the odds of choosing tadalafil were about eight times higher (odds ratio 8.04, 95% CI 4.99 to 12.96). Two studies asked partners, and they leaned the same way (odds ratio 14.50).

Two caveats keep this in proportion. Nine of the 16 studies were crossover trials and the rest were prospective observational studies, and the review authors rated study quality as low to moderate. And preference did not make men stay on treatment longer: adherence and persistence were similar for both drugs. The most likely reason for the preference is the longer window, which takes the clock out of sex.

Side effects head to head

Overall side effect rates were similar, but the pattern was different. The table and chart show the pooled odds ratios from trials that compared the two drugs directly. An odds ratio below 1 means the side effect was less common on tadalafil.

Side effect	Studies	Odds ratio, tadalafil vs sildenafil (95% CI)	Result
Flushing	10	0.39 (0.27 to 0.54)	Less common on tadalafil
Nasal congestion	7	0.69 (0.45 to 1.06)	No significant difference
Nasopharyngitis	3	0.93 (0.52 to 1.67)	No significant difference
Headache	12	0.97 (0.79 to 1.19)	No significant difference
Any side effect	5	1.09 (0.89 to 1.34)	No significant difference
Indigestion	7	1.41 (0.98 to 2.03)	No significant difference
Back pain	5	1.79 (1.06 to 3.02)	More common on tadalafil
Muscle aches	4	4.89 (1.66 to 14.43)	More common on tadalafil

Headache, the most common side effect of both drugs, was no different between them. If flushing bothers you on sildenafil, tadalafil is worth trying. If you get back pain or aching muscles the day after tadalafil, sildenafil causes those far less often.

Side effect rates on each label

For absolute numbers, each drug's own placebo-controlled trials are the reference. These come from different trials, so small gaps between the columns mean little. The large gaps, such as flushing and back pain, match the head-to-head data above.

Side effect	Sildenafil, flexible dose (n=734)	Placebo (n=725)	Tadalafil 20 mg (n=635)	Placebo (n=476)
Headache	16%	4%	15%	5%
Flushing	10%	1%	3%	1%
Indigestion	7%	2%	10%	1%
Nasal congestion	4%	2%	3%	1%
Back pain	2%	2%	6%	3%
Muscle aches	Not listed	Not listed	3%	1%
Abnormal vision	3%	0%	Not listed	Not listed

"Not listed" means the rate fell below the threshold for that label table (reported by at least 2% of patients and more often than on placebo). At a fixed 100 mg dose, sildenafil side effects are higher: headache 28%, flushing 18%, indigestion 17% and abnormal vision 11%. Our sildenafil strengths comparison charts every dose, and our high-dose tadalafil guide does the same for tadalafil.

Vision changes: why sildenafil can tint things blue

Both drugs also act weakly on PDE6, an enzyme the retina uses to process light. Sildenafil is only about 10 times more potent at PDE5 than at PDE6, which is why some men notice a blue or green tinge, light sensitivity or blurred vision for a few hours. Tadalafil is 700 times more potent at PDE5 than at PDE6, and across its clinical studies changes in color vision were reported in fewer than 0.1% of patients.

	Sildenafil	Tadalafil
Potency at PDE5 compared with retinal PDE6	About 10 times	700 times
Vision effects in trials	Abnormal vision 3% (flexible dose), 11% at 100 mg	Color vision changes under 0.1%

These color effects are temporary. Sudden loss of vision in one or both eyes is different, and it is an emergency with either drug.

Back pain and muscle aches with tadalafil

The Cialis label describes the pattern clearly. Back pain or muscle aches generally start 12 to 24 hours after a dose and usually resolve within 48 hours. The discomfort tends to affect the lower back, buttocks and thighs, is often worse lying down, and is usually mild or moderate, settling without treatment.

Tadalafil also acts on PDE11, an enzyme found in skeletal muscle, and it is often blamed for these aches. The label states that the clinical consequence of PDE11 inhibition in humans has not been defined, so that link is not proven. The practical fix is a lower dose of tadalafil or a switch to sildenafil.

Daily tadalafil: an option sildenafil does not have

Tadalafil can be taken every day at about the same time, without timing it to sex. The label starts daily dosing at 2.5 mg and allows an increase to 5 mg. Because of the long half-life, levels stay steady and the drug is always available. The 5 mg daily dose is also approved for the urinary symptoms of an enlarged prostate (BPH), so men with both problems can treat them with one tablet.

Sildenafil has no daily regimen for erectile dysfunction. For men who have sex several times a week, or who dislike planning, daily tadalafil is often the most comfortable option. Vidalista 5mg and Vidalista 2.5mg match the labelled daily doses.

Doses compared

	Sildenafil	Tadalafil
Usual start	50 mg	10 mg as needed, or 2.5 mg daily
Lower dose	25 mg	5 mg as needed
Labelled maximum	100 mg, once a day	20 mg as needed, once a day; 5 mg daily
Matching products we stock	Cenforce 50mg, Cenforce 100mg, Kamagra 100mg	Vidalista 10mg, Vidalista 20mg, Tadacip 20mg

There is no exact conversion between the two. In head-to-head trials the most common pairing was tadalafil 20 mg against sildenafil 50 mg or 100 mg. Going above either label maximum has not been shown to improve erections and does raise side effects.

Who should start lower

Situation	Sildenafil label	Tadalafil label
Age 65 and over	Consider starting at 25 mg (exposure about 84% higher)	No adjustment for age alone (exposure 25% higher)
Liver impairment	Consider starting at 25 mg	Mild or moderate: no more than 10 mg once a day. Severe: not recommended
Kidney impairment	Severe (creatinine clearance under 30 mL/min): consider starting at 25 mg	Moderate (30 to 50 mL/min): start at 5 mg, maximum 10 mg once every 48 hours. Severe or dialysis: maximum 5 mg once every 72 hours
Ritonavir, ketoconazole and other strong CYP3A4 inhibitors	Ritonavir: 25 mg maximum in 48 hours. Others, and erythromycin: consider starting at 25 mg	10 mg maximum, no more than once every 72 hours
Alpha blockers	Be stable on the alpha blocker, then start at 25 mg	Be stable on the alpha blocker, then start at the lowest dose

The age row is a real difference. Older men are more likely to be advised a reduced sildenafil dose, while the tadalafil label does not call for one based on age alone.

Nitrates: the 24 hour and 48 hour question

Both drugs are contraindicated with nitrates in any form, including nitroglycerin sprays and tablets for angina and nitrite poppers, because together they can drop blood pressure dangerously. The difference lies in what happens in an emergency. The Cialis label says that if a nitrate is judged medically necessary in a life-threatening situation, at least 48 hours should pass after the last tadalafil dose. In its nitroglycerin study, the interaction was still present at 24 hours and was not seen at 48 hours on most measures.

The Viagra label does not set a safe interval. It notes that sildenafil levels 24 hours after a 100 mg dose are about 2 ng/mL, against a peak of about 440 ng/mL, but that it is unknown whether nitrates can be given safely at that point. A review of sildenafil's cardiovascular safety (PMID 12735784) advised that nitrates should not be given within 24 hours of sildenafil. Anyone who might need nitroglycerin should settle this with their cardiologist before using either drug.

Alpha blockers and blood pressure medicines

Alpha blockers such as doxazosin and tamsulosin, used for blood pressure or prostate symptoms, add to the blood pressure lowering effect of both drugs. Both labels say to be stable on the alpha blocker first and then begin at a low dose: 25 mg for sildenafil, the lowest dose for tadalafil. Tadalafil is not recommended together with an alpha blocker when the aim is to treat BPH. Other blood pressure tablets are generally compatible, but dizziness on standing is more likely, especially with alcohol.

Which one to choose

• You want spontaneity or have sex across a weekend: tadalafil 10 mg or 20 mg as needed, or 2.5 mg or 5 mg daily.
• You have sex occasionally and prefer a short-acting drug: sildenafil 50 mg, adjusted to 25 mg or 100 mg.
• You usually eat a large meal first: tadalafil, because food does not affect it.
• Flushing bothers you: tadalafil causes less of it.
• You get back pain or aching muscles: sildenafil.
• You also have enlarged prostate symptoms: daily tadalafil 5 mg treats both.
• You are over 65: either can work; sildenafil often starts at 25 mg, while tadalafil needs no age-based change.
• You might need nitroglycerin: speak to your cardiologist before using either drug.

Switching from one to the other

Many men who do not get on with one drug do well on the other, and switching is reasonable. Never take both on the same day: the Viagra label advises against combining it with any other PDE5 inhibitor because blood pressure may fall further. Because tadalafil is still active a day or more after a dose, ask your prescriber how long to wait before switching from tadalafil to sildenafil.

Give each drug a fair trial before judging it: several attempts, at an adequate dose, taken at the right time relative to sex and to food. A single failed attempt says very little.

Generic options

Both molecules are off patent. We stock sildenafil as Cenforce and Kamagra, and tadalafil as Vidalista and Tadacip. These are Indian generics made by licensed manufacturers; they contain the same active ingredients but are not FDA approved products. Our Vidalista 20mg vs Cialis comparison, generic Cialis guide and Kamagra forms comparison cover the brands in detail.

Two more drugs belong to the same class. Our vardenafil, tadalafil and sildenafil comparison adds vardenafil, and the avanafil guide covers the newest option.

When to see a doctor

Erectile dysfunction can be an early sign of heart disease, diabetes or low testosterone, so a check-up before starting either drug is worthwhile, especially if you have not had one recently. See a doctor too if neither drug works at the labelled maximum when taken correctly, or if erections have been getting steadily worse. Our guide to erectile dysfunction causes covers the common ones.

Warning signs that need emergency care

• An erection lasting more than 4 hours.
• Sudden loss of vision in one or both eyes.
• Sudden decrease or loss of hearing, sometimes with ringing in the ears or dizziness.
• Chest pain, fainting or severe dizziness during or after sex.

All the treatments we stock are listed on our erectile dysfunction page.

References

• FDA prescribing information for Viagra (sildenafil citrate), DailyMed: sections 2 (Dosage and Administration), 4 (Contraindications), 5.7, 6.1 Tables 1 and 2, 7.5 (Alcohol), 12.1 (Mechanism of Action) and 12.3 (Pharmacokinetics)
• FDA prescribing information for Cialis (tadalafil), DailyMed: sections 2.1 to 2.7, 5 (Warnings and Precautions), 6.1 Table 1, 12.1, 12.2, 12.3 and 14 (Clinical Studies)
• Gong B, Ma M, Xie W, et al. Direct comparison of tadalafil with sildenafil for the treatment of erectile dysfunction: a systematic review and meta-analysis. Int Urol Nephrol. 2017;49(10):1731-1740. PMID 28741090
• Carson CC, Rajfer J, Eardley I, et al. The efficacy and safety of tadalafil: an update. BJU Int. 2004;93(9):1276-81. PMID 15180622
• Goldstein I, Lue TF, Padma-Nathan H, et al. Oral sildenafil in the treatment of erectile dysfunction. N Engl J Med. 1998;338(20):1397-404. PMID 9580646
• Tran D, Howes LG. Cardiovascular safety of sildenafil. Drug Saf. 2003;26(7):453-60. PMID 12735784

Medical disclaimer: this article is for information only and is not medical advice. Sildenafil and tadalafil are prescription medicines. Speak to a doctor or pharmacist before starting either, especially if you have heart disease, take nitrates, alpha blockers or other regular medicines, or have liver or kidney problems. If an erection lasts more than 4 hours or you suddenly lose vision or hearing, seek emergency care.