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Feb 11, 2022
7 min read

Viagra vs. Cialis: similarities and differences

Viagra (sildenafil) and Cialis (tadalafil) are both phosphodiesterase-5 (PDE5) inhibitors used to treat erectile dysfunction (ED). They can be effective in as little as one hour, though these medicines can work more rapidly for some people. Both cause similar side effects, like upset stomach, diarrhea, flushing, nasal congestion, headache, and changes in blood pressure.

Disclaimer

If you have any medical questions or concerns, please talk to your healthcare provider. The articles on Health Guide are underpinned by peer-reviewed research and information drawn from medical societies and governmental agencies. However, they are not a substitute for professional medical advice, diagnosis, or treatment.

People with erectile dysfunction (ED) interested in medical treatment may feel overwhelmed by the options available, especially if their healthcare provider recommends an ED medication. 

You may be wondering what the differences are between drugs like Viagra and Cialis, two commonly prescribed ED treatments. Read on to learn more about their similarities and differences. 

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What are Viagra and Cialis?

Viagra (active ingredient sildenafil; see Important Safety Information) and Cialis (active ingredient tadalafil; see Important Safety Information) are drugs commonly used to treat erectile dysfunction (ED). ED is a common sexual dysfunction that may cause difficulties getting an erection, keeping an erection, or having morning erections. It affects 30–50 million American men and can prevent people from having a satisfying sex life (Sooriyamoorthy, 2021).  

Both Viagra and Cialis are phosphodiesterase type-5 enzyme (PDE5) inhibitors. These drugs enhance blood flow to the penis, thereby improving erections. Other erectile dysfunction treatments in the PDE5 inhibitor category include vardenafil (brand name Levitra) and avanafil (brand name Stendra). 

None of these drugs are magic pills—they require arousal and sexual stimulation to work effectively. 

What is Viagra?

Viagra was the first drug FDA-approved to treat erectile dysfunction. It may also be used to treat pulmonary arterial hypertension (under the brand name Revatio; see Important Safety Information). 

Like other medications, some healthcare providers may use it “off-label” to treat conditions other than the FDA-approved ones. These conditions may include female sexual arousal disorder and Raynaud’s phenomenon. Viagra may also be used as an additional treatment for altitude-induced hypoxemia (altitude sickness) (Smith, 2021). 

What is Cialis?

Cialis is an FDA-approved treatment for erectile dysfunction and benign prostatic hyperplasia, a type of enlarged prostate. Under the brand name Adcirca, it can be used to treat pulmonary arterial hypertension. Like Viagra, some providers use it off-label to treat Raynaud’s phenomenon (UpToDate, n.d.).

How effective are Viagra and Cialis

Viagra and Cialis are both effective at treating erectile dysfunction. Clinical studies comparing the two drugs show they have comparable effectiveness at improving erections.

Cialis has the additional benefit of working when taken as needed or once daily (Gong, 2017). 

How fast do they work?

Viagra and Cialis take approximately the same amount of time to work. Let’s look at each a little more closely. 

Viagra

Most people who take Viagra notice that it starts working around an hour after taking it, but it can be effective in as little as 30 minutes. It can take up to two hours to start working for some people. The recommendation is to take Viagra about 60 minutes before sexual activity. You can take it with food or on an empty stomach. However, if you take it with a high-fat meal, it may take longer to start working (DailyMed-a, 2022; Zucchi, 2019). 

Cialis

For most people, Cialis starts working in about 30 minutes. Like Viagra, you can take it with or without food (DailyMed-b, 2022; Zucchi, 2019). 

How long do they last?

One of the key differences between Viagra and Cialis is how long they last in your body. 

Viagra

In most cases, Viagra leaves the body in about 6 to 8 hours. However, this may depend on other factors like what dose you are taking, other medications, and your overall health (DailyMed-a, 2022).

Cialis

Cialis can work for 24 to 36 hours. That’s why you can take it either “as needed” about 60 minutes before sex or once a day. The latter option can come in handy if you are unsure when sex may occur. However, just like Viagra, other factors may affect how long it lasts for you (DailyMed-b, 2022).

What doses do Viagra and Cialis come in?

The dosage for these two drugs is another area where they differ. 

Viagra

Sildenafil citrate (Viagra) comes in 25 mg, 50 mg, or 100 mg oral tablets. 

While most people start with the 50 mg dose, your healthcare provider may recommend a higher or lower dose, depending on your medical history. The maximum daily dose for Viagra is 100 mg per day.

Cialis

Tadalafil (Cialis) is available as an oral tablet in varying strengths: 2.5 mg, 5 mg, 10 mg, and 20 mg. 

When using Cialis as needed for ED, most people start with 10 mg taken at least 30 minutes before sex. However, if you are taking it daily for ED, you’ll likely start with 2.5 mg taken around the same time each day. Your healthcare provider may recommend a higher or lower dose depending on your medical history and other factors.

People using Cialis for BPH usually take a starting dose of 5 mg per day. 

The maximum daily dose for Cialis is 20 mg per day. 

Side effects of Viagra and Cialis

Since they belong to the same drug class, it is not surprising that Viagra and Cialis have similar side effects. The risk of these side effects may depend on your medication dosage, medical conditions, and other factors. 

Priapism

Priapism is a painful, prolonged erection that lasts more than 4–6 hours and is a medical emergency. If the erection is not relieved, it can lead to permanent damage to your penis. All PDE5 inhibitors, including Viagra and Cialis, increase your risk of developing priapism. Fortunately, this is an infrequent serious side effect (DailyMed-a, 2022; DailyMed-b, 2022).

Low blood pressure

PDE5 inhibitors work by encouraging the penis blood vessels to relax, allowing more blood to flow into the penis to maintain the erection. However, they affect your blood vessels throughout your body, not just those in the penis. When your vessels relax (or dilate), it causes your blood pressure to drop. 

In some cases, this is the effect we are looking for in a drug. But if you already have low blood pressure or are taking another medication to lower your blood pressure, PDE5 inhibitors can make your blood pressure drop lower than intended. If your blood pressure drops too much, the results can be fatal (DailyMed-a, 2022; DailyMed-b, 2022).

Blurred vision

Rarely, people who take PDE5 inhibitors report blurry vision due to an eye condition called non-arteritic ischemic optic neuropathy (NAION). This mainly happens in people who already have other risk factors for this condition. NAION can lead to changes in your vision that may or may not be permanent. Before starting a PDE5 inhibitor like Viagra or Cialis, talk to your eye care professional about whether you are at an increased risk for developing NAION (DailyMed-a, 2022; DailyMed-b, 2022).

Effects on the heart 

The data on the cardiovascular effects of PDE5 inhibitors is murky regarding how these drugs affect the heart. It is unclear whether any new heart issues reported in clinical trials resulted from the medications, sexual activity, or other factors. For this reason, people with preexisting risk factors for heart disease (like high blood pressure, high cholesterol, diabetes, etc.) or a history of heart attacks or strokes should get medical advice from their healthcare provider before starting Viagra or Cialis (DailyMed-a, 2022; DailyMed-b, 2022).

Aside from the rare side effects above, there are some common side effects of both drugs as well.

Viagra side effects

The most common side effects of Viagra include (Smith, 2021):

  • Headaches
  • Facial flushing
  • Indigestion/heartburn 
  • Back pain
  • Stuffy nose
  • Nausea 

Approximately 3% of people in clinical studies noted a blue or greenish tinge to their vision after taking Viagra. Fortunately, this side effect is temporary. The thought is that this happens because sildenafil can bind to PDE6 in addition to PDE5. PDE6 works in the retina, the part of your eye responsible for processing light and color. Sildenafil seems to be more likely to bind PDE6 than other PDE5 inhibitors. Having said that, this color vision change does not occur frequently (DailyMed-a, 2022; Gong, 2017).

Cialis side effects

The most common side effects of Cialis include (DailyMed-b, 2022):

  • Headaches
  • Indigestion/heartburn
  • Back pain
  • Muscle aches
  • Nasal congestion
  • Flushing
  • Pain in arms or legs

Cialis is less likely to cause facial flushing than Viagra, but more likely to cause back pain, muscle aches, and pain in arms or legs. If it occurs, the pain usually starts 12–24 hours after taking Cialis and goes away 48 hours after your last dose. People describe the back pain as diffuse lower back pain that may also affect your glutes or thighs that worsens with lying down. Fortunately, not only is this pain temporary, but it is not severe pain for most people (Gong, 2017; DailyMed-b, 2022).

Viagra vs. Cialis drug interactions

As with any drug, seek medical advice from your healthcare provider before starting erectile dysfunction medications, including Viagra or Cialis. Be sure to discuss your medical history, as well as any over-the-counter supplements or prescription medicines you are taking so you can best prevent any potential drug interactions. 

Given that both Viagra and Cialis are PDE5 inhibitors, it comes as no surprise that they have similar drug interaction warnings. 

One of the most serious drug interactions with PDE5 inhibitors, like Viagra and Cialis, happens with nitrates, drugs often used to treat chest pain (angina) or other heart conditions. Taking these drugs with PDE5 inhibitors can lead to a severe drop in blood pressure with potentially fatal consequences. Examples include nitroglycerin and amyl nitrites, also called “poppers”  (DailyMed-a, 2022; DailyMed-b, 2022).

Similarly, if you take alpha-blockers, you should not take Viagra or Cialis. Alpha-blockers are used to treat high blood pressure or some prostate issues like benign prostatic hyperplasia (BPH). 

Since alpha-blockers lower your blood pressure, combining them with PDE5 inhibitors can again lead to a dangerous situation where your blood pressure is too low. Examples of alpha-blockers include terazosin (brand name Hytrin) and tamsulosin (brand name Flomax) (DailyMed-a, 2022; DailyMed-b, 2022).

Lastly, drugs that affect the CYP3A4 enzyme system may disrupt how your liver processes Viagra and Cialis, thereby increasing your risk of side effects. Examples of these drugs include ketoconazole, itraconazole, ritonavir, indinavir, and erythromycin. Grapefruit juice can also affect this enzyme system, so drinking large quantities could theoretically affect your sildenafil or tadalafil drug levels (DailyMed-a, 2022; DailyMed-b, 2022).

There may be other potential drug interactions with Viagra or Cialis. Seek medical advice from your pharmacist or healthcare professional if you have questions or concerns. 

Differences and similarities of Viagra and Cialis

References

  1. DailyMed. (2022-a). Viagra- sildenafil citrate tablet, film-coated. Retrieved on Feb. 3, 2022 from https://dailymed.nlm.nih.gov/dailymed/lookup.cfm?setid=a2a9f459-e692-4e85-83b0-a35fbf35e91b
  2. DailyMed. (2022-b). Tadalafil tablet, coated. Retrieved on Feb. 3, 2022 from https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=abcb2cda-9b3f-45f6-ad5f-c2370b0dd114
  3. Gong, B., Ma, M., Xie, W., Yang, X., Huang, Y., Sun, T., et al. (2017). Direct comparison of tadalafil with sildenafil for the treatment of erectile dysfunction: a systematic review and meta-analysis. International Urology and Nephrology, 49(10), 1731–1740. doi:10.1007/s11255-017-1644-5. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5603624/
  4. Smith, B. P. & Babos, M. (2021). Sildenafil. [Updated June 29, 2021]. In: StatPearls [Internet]. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK558978/
  5. Sooriyamoorthy, T. & Leslie, S. W. (2021). Erectile dysfunction. [Updated Aug. 12, 2021]. In: StatPearls [Internet]. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK562253/
  6. UpToDate. (n.d.). Tadalafil: drug information. Retrieved on Feb. 3, 2022 from https://www.uptodate.com/contents/tadalafil-drug-information
  7. Zucchi, A., Costantini, E., Scroppo, F. I., Silvani, M., Kopa, Z., Illiano, E., et al. (2019). The first-generation phosphodiesterase 5 inhibitors and their pharmacokinetic issue. Andrology, 7(6), 804–817. doi: 10.1111/andr.12683. Retrieved from https://pubmed.ncbi.nlm.nih.gov/31350821/