Sildenafil and tadalafil are the two medications most often prescribed for erectile dysfunction, and they belong to the same class. Both are PDE5 inhibitors: they help you get and keep an erection by increasing blood flow to the penis during sexual stimulation, and neither causes an erection on its own. The practical difference between them is timing. Sildenafil is taken as needed, about an hour before sex. Tadalafil's effect lasts longer after a dose, and it is also prescribed as a low dose taken once a day.
The short answer
Same class, different clock. Sildenafil (the generic for Viagra®) is taken as needed, usually about an hour before sex, within a window of 30 minutes to 4 hours. Tadalafil (the generic for Cialis®) can be taken as needed, with an effect observed up to 36 hours after a dose, or as a low daily dose that isn't timed to sex at all. A clinician chooses between them based on your health history, your other medications and how you'd rather take it.
Same class, two schedules
Both medications block an enzyme called phosphodiesterase type 5, or PDE5. During sexual stimulation the body releases nitric oxide in the penis, which relaxes smooth muscle and lets blood flow in; PDE5 normally winds that signal down. Blocking it keeps the blood flow going, which is why these medications need sexual stimulation to work and why they don't increase desire.
Where they differ is how long each stays active, and that shapes how each is prescribed:
- Sildenafil is taken as needed. The label's guidance is to take it approximately 1 hour before sexual activity, though it can be taken anywhere from 30 minutes to 4 hours before, and no more than once a day. A high-fat meal can delay when it starts to work.
- Tadalafil can be taken as needed too, at least 30 minutes before sex, and its effect has been observed up to 36 hours after a dose. It can also be taken once a day at the same time every day, without regard to when you have sex, and food doesn't change how it's absorbed.
| Sildenafil | Tadalafil | |
|---|---|---|
| Generic for | Viagra® | Cialis® |
| How it's taken | As needed | As needed, or once a day |
| Timing on the label | About 1 hour before sex, within 30 minutes to 4 hours | At least 30 minutes before sex, or a daily dose at the same time each day |
| How long the effect has been observed | Within the hours after a dose | Up to 36 hours after an as-needed dose |
| Food | A high-fat meal can delay the start | No effect |
| Maximum | Once a day | Once a day |
What the label says about doses
The sildenafil label describes a usual starting dose of 50 mg, adjusted between 25 mg and 100 mg based on how it works and how it's tolerated. The tadalafil label describes 10 mg as the usual starting dose for as-needed use, and a once-daily dose of 2.5 mg to 5 mg, started at 2.5 mg. Those ranges are the clinician's to work within. Which medication and which dose are appropriate for you depends on your health history, your other medications and how you respond, and that is a prescribing decision rather than a preference you set yourself.
Side effects they share
Because they work the same way, their common side effects overlap: headache, flushing, a stuffy or runny nose, indigestion, back pain and muscle aches, and dizziness. Sildenafil is more often linked to temporary changes in color vision, such as a blue tinge to objects. Tadalafil is more often linked to back pain and muscle aches, which the label notes usually appear 12 to 24 hours after a dose and typically resolve within 48 hours.
Rare but serious effects apply to both: a sudden decrease or loss of vision or hearing, chest pain during sex, fainting, and an erection that lasts longer than 4 hours, which is a medical emergency. Those are reasons to stop and get care right away.
What they aren't combined with
Neither medication is taken with nitrates, the chest-pain medications such as nitroglycerin and isosorbide, or with riociguat; the combination can drop blood pressure to a dangerous level. Alpha-blockers for prostate symptoms or blood pressure, certain antifungal and HIV medications, and alcohol also matter to how these medications are prescribed. The full list is in our guide to what not to take with sildenafil or tadalafil. The intake asks about every medication you take so your clinician can weigh them.
How a clinician chooses
There is no better medication of the two, only a better fit. Things a clinician weighs:
- How you'd rather take it. If sex is occasional and planned, an as-needed tablet fits. If you'd rather not watch the clock, tadalafil's longer window or the daily dose may fit better. Our guide to daily tadalafil covers that option.
- Meals. Sildenafil's start can be delayed by a heavy meal; tadalafil's isn't.
- Your other medications. An alpha-blocker, a strong CYP3A4 inhibitor such as ritonavir or ketoconazole, or a blood pressure regimen can change which medication and dose are appropriate.
- Side effects you've had. A man who had vision changes on one medication, or back pain on the other, may do better on a switch.
- Cost. At Pallas both generics are priced the same; the daily option is priced separately.
At Pallas the intake asks what you'd prefer and marks a suggestion from your answers. A US-licensed clinician reviews your health history and decides whether sildenafil or tadalafil is appropriate for you, and at what dose.
What they cost at Pallas
Sildenafil (Generic for Viagra®) and Tadalafil (Generic for Cialis®) start at $25 a month on a 90-day supply, and daily tadalafil starts at $25 a month on a 180-day supply. One plan price covers the clinician review, check-ins, care-team messaging and the medication, shipped free by a US-licensed pharmacy in plain packaging. Nothing is charged until a clinician prescribes. Cash-pay only; not billed to insurance. Every plan and supply is on the erectile dysfunction treatment page.
Bottom line: Sildenafil and tadalafil work the same way and share most side effects. Sildenafil is taken as needed about an hour before sex; tadalafil's effect has been observed up to 36 hours after a dose, and it is also prescribed as a low daily dose. Neither is taken with nitrates or riociguat. Which one fits you is a clinical decision, and a clinician can switch you if the first choice isn't right.
