Sildenafil

Why there is no syringe

There is no syringe. Sildenafil is an oral film-coated tablet in strengths of 25 / 50 / 100 mg, swallowed whole, and the only "dose conversion" it involves is picking the right tablet. Naming a barrel here would invent an instrument the reader does not have. What does need converting on this drug is time, not volume: the label's lead time (approximately 1 hour before) and the window after it are the numbers that decide whether a dose works, and /tools/ed-medication-timing does that arithmetic from these same figures.

Administration

1. 1

   Rule out nitrates before anything else

   This is the step that kills people when it is skipped. “VIAGRA was shown to potentiate the hypotensive effects of nitrates, and its administration to patients who are using nitric oxide donors such as organic nitrates or organic nitrites in any form either regularly and/or intermittently is therefore contraindicated.” (section 4.1 Nitrates). That covers nitroglycerin tablets, sprays, ointments, pastes and patches, isosorbide mononitrate and dinitrate, and the recreational nitrites sold as “poppers”. Guanylate cyclase stimulators such as riociguat are contraindicated too: “Do not use VIAGRA in patients who are using a GC stimulator, such as riociguat. PDE5 inhibitors, including VIAGRA, may potentiate the hypotensive effects of GC stimulators.” (section 4.3 Concomitant Guanylate Cyclase (GC) Stimulators).

2. 2

   Check the interval before any nitrate is given

   Label states none — "it is unknown". “After patients have taken VIAGRA, it is unknown when nitrates, if necessary, can be safely administered.” (section 4.1 Nitrates). Two of the four labels name an interval that must elapse before a nitrate can even be considered in a life-threatening situation: 12 hours for avanafil, 48 hours for tadalafil. The sildenafil label says it is unknown, and the vardenafil label says it has not been determined. None of the four says a nitrate is ever safe alongside a dose.

3. 3

   Take it the lead time the label gives

   “For most patients, the recommended dose is 50 mg taken, as needed, approximately 1 hour before sexual activity. However, VIAGRA may be taken anywhere from 30 minutes to 4 hours before sexual activity.” (section 2.1 Dosage Information). Note what that figure is: dosage section instruction; 30 min to 4 hr permitted. No label of the four states an “onset of action”, so nothing on this page presents one.

4. 4

   Account for food

   A high-fat meal pushes the peak back a full hour and cuts it by 29% — the largest food effect of the four. “When VIAGRA is taken with a high fat meal, the rate of absorption is reduced, with a mean delay in Tmax of 60 minutes and a mean reduction in Cmax of 29%.” (section 12.3 Pharmacokinetics).

5. 5

   Understand what the four-hour figure is

   The VIAGRA label permits dosing anywhere from 30 minutes to 4 hours before sexual activity, and separately reports a RigiScan time-course study showing “an effect for up to 4 hours but the response was diminished compared to 2 hours” (section 12.2 Pharmacodynamics). Those are two different four-hours — one is a dosing window, the other is how long the effect was still measurable. The useful part of the window is front-loaded.

6. 6

   Do not repeat the dose the same day

   “The maximum recommended dosing frequency is once per day.” (section 2.1 Dosage Information). The label states an effect lasting up to 4 hours. Effect is documented to 4 hours but was weaker at 4 hours than at 2 — the useful window is front-loaded.

Storage

• —Store at 25 °C (77 °F); excursions permitted to 15–30 °C (59–86 °F) [see USP Controlled Room Temperature] — quoted from section 16 of the VIAGRA label.
• —Keep the tablets in the original container, out of reach of children.
• —Tablets ordered from an unlicensed overseas seller are outside all of this. FDA has repeatedly found products sold as PDE5 inhibitors that contain undeclared active ingredient at undeclared strengths; nothing on this page describes those.

Monitoring

Frequently asked questions

What dose does the FDA label actually state?

The Viagra label's dosage section states: “For most patients, the recommended dose is 50 mg taken, as needed, approximately 1 hour before sexual activity.” It then states: “Based on effectiveness and toleration, the dose may be increased to a maximum recommended dose of 100 mg or decreased to 25 mg.” and “The maximum recommended dosing frequency is once per day.” The approved as-needed range is therefore 25–100 mg, and the tablet strengths supplied are 25 / 50 / 100 mg.

How long before sex should it be taken, and how long does it last?

The label says to take it approximately 1 hour before, and be clear what that figure is — the basis is dosage section instruction; 30 min to 4 hr permitted. For duration: The label states an effect lasting up to 4 hours. Effect is documented to 4 hours but was weaker at 4 hours than at 2 — the useful window is front-loaded. “The time course of effect was examined in one study, showing an effect for up to 4 hours but the response was diminished compared to 2 hours.” (section 12.2 Pharmacodynamics). None of the four labels states an “onset of action”; they state a lead time or a time-to-peak, which are different quantities.

Can it be taken with nitrates if enough time has passed?

Label states none — "it is unknown". The label's own words: “After patients have taken VIAGRA, it is unknown when nitrates, if necessary, can be safely administered.” (section 4.1 Nitrates). 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. Where a label declines to name a safe interval, that is not the same as saying none is needed — it is the label saying the answer is not known, and this page does not fill it in.

What if I am already on an alpha-blocker for my prostate?

Start at 25 mg; be stable on the alpha-blocker first. The label states: “When VIAGRA is co-administered with an alpha-blocker, patients should be stable on alpha-blocker therapy prior to initiating VIAGRA treatment and VIAGRA should be initiated at 25 mg” (section 2.3 Dosage Adjustments in Specific Situations). Section 7.2 of the same label words it as "VIAGRA should be initiated at the lowest dose" without naming 25 mg. The dosage section is the more specific of the two and is what this chart prints. Both drugs lower blood pressure, so the combination is a dose decision for your prescriber, not an adjustment to make on your own.

Is Revatio the same drug as Viagra?

Same molecule, different approved product. Sildenafil is approved twice: as VIAGRA (NDA 020895) in 25, 50 and 100 mg tablets for erectile dysfunction, and as REVATIO (NDA 021845) as a 20 mg tablet for pulmonary arterial hypertension, with a separate oral suspension and injection. They are not interchangeable — the indication, the dosing schedule and the strengths are all different, and the PAH product is taken three times a day. Both carry the same absolute nitrate contraindication.

Why does a heavy meal matter so much with sildenafil?

Because it has the largest food effect of the four. The VIAGRA label states: “When VIAGRA is taken with a high fat meal, the rate of absorption is reduced, with a mean delay in Tmax of 60 minutes and a mean reduction in Cmax of 29%.” A full hour of delay against a labelled lead time of about one hour is the difference between a dose that has peaked and one that has barely started. Tadalafil is the opposite case: its label states absorption is not influenced by food at all.

Does this page tell me which one to take?

No. It quotes one label. Choosing between the four is a clinical decision that turns on what else you take — nitrates and guanylate cyclase stimulators rule all four out absolutely — on your cardiovascular status, on whether you want a tablet for an occasion or one every day, and on cost. The side-by-side comparison of all four labels is at /charts/ed-medication-comparison-chart.

Sources

1. [1]FDA label — VIAGRA (sildenafil citrate) tablet, film coated, Pfizer Laboratories Div Pfizer Inc (DailyMed SPL 0b0be196-0c62-461c-94f4-9a35339b4501). Dosage, nitrate contraindication, alpha-blocker dosing, pharmacokinetics and storage all quoted from this document. Drugs@FDA: VIAGRA is NDA 020895 (Viatris), 25, 50 and 100 mg tablets, all marketed.
2. [2]FDA label — REVATIO (sildenafil) tablet, for pulmonary arterial hypertension (Drugs@FDA NDA 021845). The second approved sildenafil product, at 20 mg three times daily, confirming that the molecule carries two distinct approvals with different strengths and indications
3. [3]Drugs@FDA: FDA-Approved Drugs — the searchable record of every approved application, used to confirm the application number, the approved strengths and the marketing status of each brand named on this page

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