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Although sildenafil citrate has been found to have a number of promising medicinal properties, the only other brand-name medicine in which it is the active ingredient is Revatio, which was also developed by Pfizer. Revatio is prescribed to treat pulmonary arterial hypertension, or PAH, a form of high blood pressure that is localized to the arteries that supply the lungs.

PAH is a progressive disease with symptoms that tend to grow worse over time.

The symptoms of PAH can include chest pain or angina; dizziness and/or fainting; fatigue; rapid, hard, or irregular heartbeats; swollen abdomen; and swelling of the ankles and legs, also known as edema.

Shortness of breath is the single most widely observed symptom of PAH, which Revatio can relieve by dilating the arteries that supply blood to the lungs, thus lowering blood pressure. Revatio is available in 5- and 20-milligram tablets, as an oral suspension powder that is mixed with water, and in injectable form as well.

Generic formulations of the drug are also available. As with Viagra, Revatio and its generic substitutes are incompatible with nitrate-based drugs. Taken together, they can cause blood pressure to drop precipitously.

Generic substitutes for brand-name medications must first win the approval of the FDA before they can be marketed. Generic drugmakers submit an abbreviated new drug application, or ANDA, that is similar in most respects to the New Drug Applications, or NDAs, submitted for new and previously unmarketed prescription drugs.

NDAs must be accompanied by data from clinical trials designed to show that the proposed new drug achieves its medical goals with minimal adverse side effects. ANDAs do not require all this clinical trial data, because the drug’s efficacy and safety have already been established by the manufacturer of the brand-name drug.

However, the FDA has certain criteria that must be met by any would-be generic drug manufacturer. The proposed generic must contain the same active ingredient at the same strength(s) as the brand-name drug. It must also use the same dosage form — tablet, capsule, or liquid — and the same route of administration, whether oral or injectable.

However, generic drugs need not contain the same inactive ingredients as the brand-name drug for which they are offered as substitutes, but the FDA reserves the right to deny an ANDA if it disapproves of the inactive ingredients used in a generic. Inactive ingredients typically serve as filler material and coloring agents.

Because they are inactive, these substances are inert chemically and unlikely to cause any ill effects in those taking the drugs. Thus, all generic Viagra must have sildenafil citrate as an active ingredient and must be offered in the same dosage strengths as the brand-name drug — 25-, 50-, and 100-milligram tablets.

Because the generics’ inactive ingredients can be different from those in the branded drug, generics might look nothing at all like the latter, apart from being available in tablet form.

The first generic formulations of Viagra are scheduled to hit the U.S. market beginning in December 2017. All will be marketed under the name of Viagra’s active ingredient — sildenafil citrate.

Teva Pharmaceuticals USA, the American subsidiary of an Israeli-based drugmaker that specializes in generic drugs, is expected to bring out the first generic version, which could be followed closely by another from Mylan.

Both of these generics have been tentatively approved by the FDA. Such tentative approvals usually don’t become final until the brand-name drug’s patent has expired. In these cases, Pfizer’s patent on Viagra is good until April 2020, but the company has negotiated special licensing deals with both Teva and Mylan that will allow them to introduce generic formulations of the little blue pill early.

In return for early generic rights, Teva and Mylan will have to pay licensing fees to Pfizer until the latter’s patent finally expires in 2020.

As previously explained, the active ingredients in Viagra and the other oral ED medications are designed to temporarily restore erectile function in men who are suffering from insufficient blood flow to the penis.

This is by far the most common cause of ED, but it’s not the only cause. If your impotence has its origin in emotional or mental issues and not vascular insufficiency, then it’s unlikely that Viagra will work for you regardless of when you take it.

The same is true of ED that’s caused by traumatic or congenital injury to the blood vessels that supply the male genitals. However, if your erection problems are caused by compromised blood flow to the penis because of atherosclerosis or other physiological causes, Viagra’s active ingredient should kick in as early as 30 minutes after you take it.

If you haven’t responded within 60 minutes after taking the drug, it is possible that the drug won’t work for you. If you’ve tried the drug two or three times without success, you might want to explore other treatment options with your doctor.

That’s a difficult question to answer because all “herbal Viagra” is not the same. And some so-called herbal Viagra isn’t herbal at all. Because this sort of remedy falls under the very loose heading of dietary supplements, it is not subject to the same strict standards as prescription drugs. When the Food and Drug Administration is advised that a specific dietary supplement has caused problematic reactions, it investigates and usually issues a warning to consumers.

It may also seek to pressure the supplement’s distributor to recall the product. Some of these so-called remedies have been found to contain dangerously high levels of herbal ingredients that sometimes cause adverse side effects in humans. Such herbs include epimedium, more commonly known as horny goat weed, and yohimbine, a crystalline compound derived from the bark of the yohimbe tree. These ingredients are not bad in and of themselves, but some users may be hypersensitive to their effects and find them hard, if not impossible, to tolerate.

In other supplements, testing has determined that they contain varying but unlisted amounts of one or more of the active ingredients found in prescription ED medications. In some cases, the amount of the active ingredient is too little to be effective, while in others it is so high that it could pose a danger.

While the negative publicity caused by one of these FDA consumer warnings may persuade a supplement distributor to withdraw that specific product, it may reappear not long thereafter under a different brand name. For this reason, you should exercise special care in buying and taking dietary supplements. It’s best to purchase only those produced by a well-known and reputable supplement manufacturer. If in doubt, ask your doctor.