Viagra, Cialis, Levitra, and Stendra are oral medications used to treat erectile dysfunction (ED). You may also know them by their generic names:

About 30 million American men occasionally have a problem with getting or keeping an erection, according to the Urology Care Foundation. When ED becomes a problem, many men turn to these oral ED medications. They often help address the issue.

The medications work in similar ways. However, there are also some key differences, such as when you take them, how long they work, and what their side effects are.

Viagra, Cialis, Levitra, and Stendra are all in a class of drugs called PDE5 inhibitors. These drugs work by blocking an enzyme called phosphodiesterase type 5.

They also boost a chemical in your body called nitric oxide. This action encourages the muscles in your penis to relax. Relaxed muscles allow blood to flow freely so that when you’re aroused, you can get an erection. It also helps you maintain the erection long enough to have sex.

Here are the basic features of each of these drugs:

Brand nameViagraCialisLevitraStendra
What’s the generic name of this drug?sildenafiltadalafilvardenafil avanafil
Is a generic version available?Yesyesyesno
What form does it come in?oral tabletoral tabletoral tabletoral tablet
What strengths does it come in?25 mg, 50 mg, 100 mg2.5 mg, 5 mg, 10 mg, 20 mg 5 mg, 10 mg, 20 mg50 mg, 100 mg, 200 mg
What’s the typical dosage?50 mg10 mg (when used as needed); 2.5 mg (when used daily)10 mg; 5 mg (for men 65 years old and up)100 mg
When do I take it?30-60 minutes before sex30 minutes before sex 60 minutes before sex15 minutes before sex (for 100 mg and 200 mg); 30 minutes before sex (for 50 mg)
How long does it work?4 hoursup to 36 hours4-5 hours6 hours
How do I store it?Around room temperature, between 68°F to 77°F (20°C to 25°C)At 25°C (77°F)At 25°C (77°F)Around room temperature, between 68°F to 77°F (20°C to 25°C)

The drugs are available in a variety of doses, ranging from 2.5 milligrams (mg) to 200 mg. All can be taken with or without food. However, taking these drugs after eating a high-fat meal does slow the rate of absorption.

Most of them stay in your bloodstream for about four to five hours. Cialis is the exception, as it remains in your bloodstream for up to 36 hours. The length of time a drug stays in your system may be important if you’re taking other medications.

You shouldn’t take any of these drugs more than once in a 24-hour period.

Viagra, Cialis, Levitra, and Stendra are usually stocked at most pharmacies. In general, most health insurance companies won’t cover their costs. However, if you have certain medical conditions, your health plan may pay for the drug with prior authorization.

The exact price you pay for any drug will depend on your insurance plan and your pharmacy.

Generic versions of Viagra, Cialis, Levitra can cost half as much as their brand-name counterparts, if not less.

The side effects of these medications are mostly similar. Most men have only mild side effects.

Below are the drugs’ most common side effects, according to their manufacturers:

Side effectViagraCialisLevitraStendra
Stuffy or runny nosexxxx
Headachexxxx
Dizzinessxx
Upset stomachxxx
Nauseax
Indigestionxx
Vision changesx
Rashx
Flushingxxxx
Back painxxxx
Pain in the limbsx
Muscle achesxx
Sore throatx

Tell your doctor if you have any side effects that linger and don’t go away on their own.

If you have an erection that lasts longer than four hours, call your doctor right away. This condition, known as priapism, is a risk associated with all of these ED drugs.

Unlike Viagra and the other PDE5 inhibitors, Cialis is also approved to treat enlarged prostate.

Both Viagra and Cialis can be taken 30 minutes before sexual activity. However, Cialis lasts much longer and is noteworthy for the amount of time it remains in your body. You may feel the effects of the drug up to 36 hours after you take it.

The fact that it comes in a low-dose (2.5 mg) version also means that Cialis can be taken every day. A daily dose will ensure that the drug’s always in your system.

If you take Cialis, there’s a chance of limb pain. This side effect isn’t associated with any other oral ED drugs.

Viagra can take 30 to 60 minutes to work in the body, while Levitra takes 60 minutes. The effects of both drugs last for about 4 hours.

Levitra does have fewer common side effects than Viagra, and it isn’t associated with a rash or muscle aches. While vision changes are considered a common side effect of Viagra, changes to color perception are only a rare side effect of Levitra.

Stendra is the newest drug on the market, and no generic version is available for sale yet. A hallmark of Stendra is its fast-acting nature. The 100-mg and 200-mg doses can be taken as shortly as 15 minutes before sexual activity.

As a second-generation medication, Stendra also seems to have milder side effects than Viagra and the other PDE5 inhibitors that came before it. Side effects commonly caused by Viagra — but not Stendra — include dizziness, vision changes, nausea, and muscle aches.

The only common side effect caused by Stendra but not Viagra is a sore throat.

Each drug comes with the risk of drug interactions. Since PDE5 inhibitors work on the body in similar ways, Viagra, Cialis, Levitra, and Stendra lead to similar interactions.

All four of these drugs interact with:

Drinking excessive amounts of alcohol while on any PDE5 inhibitor should be avoided, and you shouldn’t combine different ED drugs.

Cialis may also be less effective if used alongside antiseizure medications such as carbamazepine (Tegretol) and phenobarbital.

For specifics on which medications are safe for you to use, it’s best to consult your doctor or pharmacist.

In some cases, using these medications may mean you need to completely avoid PDE5 inhibitors. In other cases, adjusting your dosage of the medication can reduce the possibility of drug interactions while using PDE5 inhibitors.

If you have ED, talk to your doctor about Viagra, Cialis, Levitra, and Stendra. Tell your doctor if you take other over-the-counter or prescription drugs or supplements. Be sure to mention all other health conditions you have.

When used correctly, each of these drugs has been shown to help men with ED. Take them exactly as your doctor prescribes. If you have questions or concerns, talk with your doctor.

All of them lead to good results, but it may take a little time and patience to get it right. If one drug doesn’t work or produces unpleasant side effects, you can try another drug.

It may also take some trial and error to find the dosage that works best for you. If you’re not sure that drug treatment is right for you, you can also give natural ED treatments a try.