Two clocks, not two winners
Sexual stimulation releases nitric oxide in the penis. That raises cGMP, a messenger that lets blood flow in. Both drugs slow the breakdown of that messenger. At recommended doses, neither works without stimulation.
| Feature | Viagra (sildenafil) | Cialis (tadalafil), as needed |
|---|---|---|
| Usual start | 50 mg | 10 mg |
| Dose may move to | 25 mg or up to 100 mg | 5 mg or up to 20 mg |
| When you take it | About 1 hour before sex; from 30 minutes to 4 hours before | Before anticipated sexual activity |
| What was measured | One study: effect up to 4 hours, weaker than at 2 hours | Improves function versus placebo up to 36 hours after the dose |
| How often | Once per day at most | Not more than once per day |
| Daily ED option | No labeled daily ED regimen | Yes — 2.5 mg to 5 mg once daily |
| Half-life | About 4 hours (drug and main metabolite) | About 17.5 hours (mean terminal) |
| Food | With or without food; a high-fat meal delays the peak | Without regard to food |
| Stimulation | Required | Required |
| Nitrates | Contraindicated | Contraindicated |
A 100 mg Viagra tablet and a 20 mg Cialis tablet are the tops of two different ladders. The milligrams are not a shared scale.
Sildenafil and its main metabolite each have a terminal half-life of about 4 hours, which is how long the blood level takes to fall by half. Keep that separate from the dosing window and from the study that still saw an effect at 4 hours. An erection still up after 4 hours is the emergency.
The mean terminal half-life of tadalafil is about 17.5 hours in healthy subjects. That longer clearance is why the as-needed Cialis span reaches 36 hours versus placebo, and why the Cialis label's life-threatening nitrate wait is at least 48 hours after the last dose under close medical supervision. The 36-hour figure is still a stimulation window, not a day-and-a-half erection, and not a reason for another tablet the next morning.
What dinner does to the peak
You may take Viagra with or without food. A high-fat meal still changes the arrival: mean time to the peak blood level (Tmax) is delayed by 60 minutes, and the mean peak (Cmax) falls by 29%. Eat lighter near the dose, or keep the heavy meal and expect the peak about an hour later than on an empty stomach. Taking the tablet as that plate arrives does not cancel the delay, because the delay comes from sharing the meal. If sex is soon after a rich dinner, move the dose earlier than the usual hour. Do not take a second tablet because the first feels late. The ordinary sequence is on how to take sildenafil.
Cialis may be taken without regard to food. You do not rearrange dinner to protect a tadalafil peak. Strong CYP3A4 inhibitors raise exposure for both drugs; on the Cialis label that list includes drugs such as ketoconazole and ritonavir, and grapefruit juice is named as likely to increase tadalafil exposure. Alcohol in substantial amounts can add to a blood-pressure drop with Cialis. Neither point replaces the nitrate stop.
A daily Cialis dose is a different prescription
Once-daily Cialis for erectile dysfunction starts at 2.5 mg, at about the same time every day, whether or not sex is planned that day. It may be increased to 5 mg. The daily schedule still needs arousal; the tablet does not create an erection on its own.
Skip stacking: no as-needed Cialis on top of the daily one, and no sildenafil with tadalafil on the same calendar day. Viagra's label has not studied those combinations, notes a further blood-pressure drop as a risk, and does not recommend them. That ban covers Revatio and other sildenafil products for pulmonary arterial hypertension — different illness, different doses.
Sildenafil's once-per-day rule is only a ceiling on as-needed doses. Monday and Tuesday can each have one dose. A standing morning sildenafil tablet, taken whether or not sex is likely, is not a labeled regimen. "Once a day" does not mean the same thing on the two labels.
Five milligrams once a day is also the Cialis dose for benign prostatic hyperplasia. The erectile-dysfunction daily start remains 2.5 mg. Cialis for BPH is not recommended with an alpha-blocker; if you move from an alpha-blocker to daily Cialis for BPH, the label wants the alpha-blocker stopped at least one day beforehand. Daily versus as-needed tadalafil, week to week, is in tadalafil daily versus on demand.
Stops that apply to both
Do not take either drug with nitrates. Viagra is contraindicated with nitric oxide donors such as organic nitrates or organic nitrites in any form, because it deepens the blood-pressure drop. Cialis is contraindicated with organic nitrates used every day or only sometimes. Pressure can fall suddenly to an unsafe level, with dizziness, fainting, or even heart attack or stroke. That shelf includes nitroglycerin tablets, sprays, and patches, other organic nitrates, and nitrites sold as poppers. Both labels also contraindicate guanylate cyclase stimulators such as riociguat for the same blood-pressure reason. That stop is separate from the nitrate rule.
If chest pain might send you to nitroglycerin, settle that plan with the prescriber before you fill either drug. The labels diverge on timing after a dose. For Cialis, if a nitrate is medically necessary in a life-threatening situation, wait at least 48 hours after the last dose and only under close medical supervision. For Viagra, it is unknown when nitrates can be safely administered, including at 24 hours.
An erection lasting longer than 4 hours needs emergency care. Viagra's label notes erections longer than 4 hours, and painful ones longer than 6 hours, reported infrequently. Untreated priapism can damage tissue. Use either drug with caution if you are prone to priapism. Both labels also caution use when the penis is anatomically deformed, including angulation, cavernosal fibrosis, or Peyronie's disease.
Stop and get care for sudden loss of vision in one or both eyes, which can be NAION (non-arteritic anterior ischemic optic neuropathy). A prior episode calls for caution. Stop and get prompt care for a sudden decrease or loss of hearing; both labels note that event can come with ringing and dizziness.
Be stable on an alpha-blocker before adding either drug for ED, because the combination can lower pressure further. For Cialis used for ED with an alpha-blocker, the label starts at the lowest Cialis dose. Cialis for BPH plus an alpha-blocker is a different rule, covered above.
Both labels list headache, flushing, indigestion, nasal congestion, back pain, and muscle ache among reactions seen in at least 2% of people in the labeled studies. The profiles still diverge: Viagra's common list also includes abnormal vision, and a temporary color tinge is a different event from sudden loss of sight. Cialis's common list also includes pain in limb, and delayed back or muscle ache is a frequent reason people ask to switch away from tadalafil. More on those reactions is on sildenafil side effects.
What you pay for, and how a real fill works
You pay for a clock, not for a winning brand. The number comes from the pharmacy's quote for the prescription in your hand.
As-needed sildenafil is one tablet for a planned window, at most one a day. As-needed tadalafil is also one tablet a day at most, but Friday and Saturday can be a single tadalafil tablet instead of two sildenafil tablets. Fewer tablets are not automatically a lower bill. Brand versus generic, strength, quantity, and insurance all move the quote. Once-daily tadalafil is a tablet every day, including days without sex: thirty daily tablets and four as-needed tablets are different purchases even when both are tadalafil.
Approved generic sildenafil is the same active ingredient as Viagra, and approved generic tadalafil matches Cialis. Both require a US prescription. FDA guidance says a safer online pharmacy always requires a doctor's prescription, lists a physical US address and phone number, has a licensed pharmacist, and is licensed by a state board of pharmacy. Warning signs include no prescription, no license, no pharmacist, medicine that looks different or arrives damaged, in a foreign language, undated, or expired, and prices that seem too good to be true. Some of those sites ship the wrong amount, the wrong ingredient, or something harmful.
FDA BeSafeRx is where you look up a state-licensed online pharmacy. If it is not listed, do not use it. The warning signs are in the FDA's guide to buying medicines safely online. The dose and stop language on this page was checked against the U.S. Viagra and Cialis DailyMed labels.
Match the tablet to the week you have
If sex is usually one evening you can see coming, as-needed sildenafil usually fits that plan. Take it about an hour before, inside the 30-minute to 4-hour window, and move it earlier if dinner will be high in fat.
If you want the option later that day or the next day without a second tablet, as-needed tadalafil is the long window. You still need stimulation, and you still stop at one dose that day.
If the hard part is remembering to time a pill because sex is frequent, ask about once-daily tadalafil, 2.5 mg at the same time every day, raised to 5 mg only if you and the prescriber agree. That is not a sildenafil schedule, and it is not an extra weekend tablet piled on the daily dose.
Nitrates, poppers, or riociguat end the comparison. So does sudden vision loss, sudden hearing loss, or an erection that would not go down, until a prescriber has heard it. Kidney disease, liver disease, and medicines such as ritonavir change the dose. The prescribing visit matches the drug and the strength to the planned hour, the long as-needed window, or the same-time-every-day tablet you actually need.
Frequently Asked Questions
Which is better, Viagra or Cialis?
Neither for every person. The labels do not crown one drug, and 100 mg versus 20 mg is not a score. Viagra fits a planned short window. As-needed Cialis fits a span of up to 36 hours after one dose. Once-daily Cialis fits when you do not want to time the tablet to sex. If both clocks fit and nitrates are not in the picture, the prescribing visit breaks the tie.
How long does Cialis last compared with Viagra?
Take Viagra about an hour before sex, anywhere from 30 minutes to 4 hours before. One study still measured an effect up to 4 hours later, weaker than at 2 hours. The half-life of about 4 hours is the blood level falling by half. An erection still present after 4 hours is an emergency. As-needed Cialis improves erectile function compared with placebo for up to 36 hours after one dose. Tadalafil's mean terminal half-life is about 17.5 hours. Stimulation can work in that span. A second tablet that day is not allowed. On the daily prescription, Saturday's tablet is the next dose, not the tail of Friday's.
Can you take Viagra or Cialis every day?
Cialis, yes, on the once-daily prescription: 2.5 mg for erectile dysfunction at about the same time each day, possibly increased to 5 mg. Five milligrams daily is also a urinary dose for benign prostatic hyperplasia, not the automatic ED start. Viagra stays as needed, once per day at most. Tomorrow can have its own tablet. A morning tablet just in case, or a second tablet after a heavy dinner, is outside that rule. Do not combine it with tadalafil the same day.
Does food change Viagra or Cialis?
A high-fat meal delays Viagra's peak by about 60 minutes and lowers it by about 29%, even though you may take the tablet with food. Lighter food keeps the usual hour. A heavy meal means you allow the extra time. The first bite of that meal is not a workaround. Cialis may be taken without regard to food. Dinner does not change the once-a-day cap on either drug.
Does alcohol change either drug?
Substantial alcohol with Cialis can add to a blood-pressure drop. Both labels already bar nitrates and poppers. A drink does not cancel those stops, and it is not a reason to take a second tablet.
If I took Cialis yesterday, when can I try Viagra?
Not the same day. The labels do not study stacking PDE5 inhibitors, and they advise against combining them. After as-needed tadalafil, the long half-life and the 36-hour stimulation window mean the next calendar morning is not automatically a clear slot for sildenafil. After sildenafil, wait for the next day's plan only with the prescriber's timing, not by inventing a second dose because the first felt late. Switching is a prescribing decision, not a home ladder.
Can you buy Viagra or Cialis online safely?
Yes, with a valid prescription, from a pharmacy that requires one, lists a physical US address and phone, has a licensed pharmacist, and holds a state-board license. Skipping the prescription is the FDA's first warning sign. Look the pharmacy up on BeSafeRx and skip any shop the lookup does not list. The dispensed drug should be the sildenafil or tadalafil on your prescription.