What is sildenafil and how does it work?

A  PDE-5 inhibitor, sildenafil works

by preventing the chemical process that lets blood leave an erect penis

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Disclaimer: This information isn’t a substitute for professional medical advice, diagnosis, or treatment. You should never rely upon this article for specific medical advice. If you have any questions or concerns, please talk to your doctor.


  • Sildenafil belongs to a family of drugs called PDE-5 inhibitors.
  • Sildenafil works by preventing the chemical process that lets blood leave an erect penis.
  • Sildenafil is the chemical found in Viagra
  • Generic sildenafil is usually cheaper than brand-name Viagra

Sildenafil is the active ingredient in Viagra. Viagra (sildenafil), Cialis (tadalafil), Levitra (vardenafil), and generic sildenafil are all part of a family of drugs called PDE-5 inhibitors. These drugs are used to treat erectile dysfunction (ED). Erectile dysfunction medication works by preventing the chemical process that lets blood leave an erect penis. PDE-5 inhibitors tip the chemical balance in the penis to favor getting and maintaining an erection.

Viagra vs. generic sildenafil quick facts


  • FDA approved to treat erectile dysfunction
  • Available in 25mg, 50mg, and 100mg tablets
  • Nicknamed “The Little Blue Pill
  • Costs more than generic ED medication

Generic sildenafil

  • FDA approved to treat pulmonary hypertension, but can also be used to treat erectile dysfunction
  • Each tablet contains 20mg of sildenafil, but prescriptions are available in 20, 40, 60, 80, 100 mg doses (you just take more than one pill at a time)
  • Costs significantly less than name-brand Viagra

The difference between Viagra and generic sildenafil: price, dose size, and use

The biggest difference between Viagra and generic sildenafil is the amount of active ingredient in each pill. The other main difference is that generic sildenafil is FDA approved to treat pulmonary hypertension (that’s high blood pressure in the blood vessels of the lungs) while Viagra is FDA approved to treat erectile dysfunction. It’s a little confusing since both medications use the same active ingredient, but Viagra and sildenafil work in the same way. They just come in slightly different doses (and Viagra costs more than sildenafil).

Prescribing generic sildenafil for ED

Generic sildenafil is only FDA approved to treat pulmonary hypertension. But doctors can prescribe sildenafil for any reason, and since sildenafil is exactly the same ingredient as Viagra, a lot of doctors use generic sildenafil to treat ED. This is technically considered “off-label” use of a prescription medication, but the only difference is the size of the dose in each tablet (20 mg vs. 25, 50, or 100 mg). Learn more about off-label prescribing here.

Generic Viagra also costs a fraction of what Viagra costs. And that discount can help guys treat ED who can’t afford Viagra.

Adjusting your dose: Viagra vs. sildenafil

The starting dose of Viagra is typically 50 mg. This lets doctors raise or lower a patient’s dose to achieve the desired effect while avoiding any potential side effects. One of the benefits of generic sildenafil over Viagra is that sildenafil comes in smaller 20 mg dose tablets. This lets you and your doctor “dial-in” your ideal dose of medication more carefully than the three dosage options available with Viagra (25mg, 50 mg, 100 mg)—and precision matters when it comes to erectile dysfunction medication.

Example generic sildenafil prescription: increasing your dose

  1. Your doctor writes a 40 mg prescription of generic sildenafil. (That’s two tablets of sildenafil, taken at least one hour before sexual activity).
  2. After several tries, you don’t get a “satisfactory” erection.
  3. Your doctor can gradually increase your dose to 60 mg (3 tablets), 80 mg (4 tablets), or the maximum daily dose of sildenafil—100 mg (5 tablets).

Example generic sildenafil prescription: avoiding side effects

  1. Your doctor writes a 40 mg prescription of generic sildenafil.
  2. You find that a 20 mg dose (1 tablet) gives you the erection you want. Even better, that lower dose doesn’t come with any annoying side effects like a headache or nausea. Your doctor can easily lower the prescription to 20 mg.

These fine-tuned shifts aren’t as easy with Viagra. A physician can only jump from 50 mg to 100 mg if the medication isn’t effective. And they can only drop from 50 mg to 25 mg if side effects are a problem. Jumping from 50–100 mg of Viagra is a lot compared to the small changes available with generic sildenafil.

Viagra vs. sildenafil: customize your ED treatment

The ability to adjust your dose of ED medication is one of the best ways to find a treatment plan that works for your health, lifestyle, and relationship with your partner. The flexibility of 20mg doses even allows for occasional jumps in dosage (from 20-40mg) to give a man a bit more of what he (and his partner wants). This increase is particularly helpful for dealing with:

  • Performance anxiety with a new partner
  • Combating the distractions and loss of “erotic focus” from stress

Moreover, the ability to achieve a robust erection can create a positive feedback loop that allows a man to focus on his—and his partner’s—experience and not worry about getting or maintaining an erection. However, you should never increase your dose without consulting your physician.


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