Erectile Dysfunction
Updated: Nov 10, 2021
Oral ED Medications: An Overview
Published: Sep 22, 2020

Sildenafil and tadalafil are oral medications prescribed to treat erectile dysfunction or ED. ED is a common problem that affects as many as 30 million men (NIH Consensus Development Panel, 1993). The prevalence increases with age. ED has many causes, including decreased blood flow to the penis, stress or emotional factors, or as symptomatology of heart disease, high blood pressure, or diabetes. The first step in treating ED is for a health care provider, such as the licensed physicians at Invigor Medical, to evaluate your overall health and ED history.

Treatment Options for ED

Treatment options for ED include the following:

  • Oral ED medications that block phosphodiesterases such as sildenafil and tadalafil
  • Testosterone injections if testosterone levels are low
  • Penile injections such as Trimix
  • Vacuum erection devices
  • Penile implants and other surgical solutions

The relationship between nitric oxide, phosphodiesterase, and ED

An erection is the result of increased blood flow into the penis. Sexual stimulation increases blood flow into the penis, trapping it temporarily in spaces called the corpus cavernosum. These spaces make up the bulk of the structure of the penis and surround the urethra, the tube shared by both the urinary and the reproductive systems.

Small blood vessels entering the penis are lined with smooth muscle that contracts and relaxes to decrease or increase blood flow through them. Damage to small blood vessels from diabetes, high blood pressure, or lipid deposits can reduce blood flow. When blood enters the penis and fills the corpus cavernosum, the swollen tissue presses on small blood vessels draining the penis. This process results in vasocongestion or holding blood in the penis. The pressure within the penis increases, and it becomes rigid (Lyseng-Williamson & Wagstaff, 2002).

Nitric oxide and its effect on blood vessels

Nitric oxide is a small molecule that is found throughout the body. It is released from nerve endings near the small blood vessels in the corpus cavernosum of the penis after sexual stimulation. Nitric oxide triggers a chemical cascade with the end result of dilating blood vessels and increasing blood flow into the penis, causing an erection. cGMP (another chemical in the cascade) levels must remain high to maintain the erection. Phosphodiesterase inactivates cGMP. Inactivating cGMP allows the smooth muscles in the walls of the small blood vessels to contract, decreasing blood flow and ending the erection.

Oral ED medications, such as sildenafil and tadalafil, are phosphodiesterase inhibitors. By inhibiting phosphodiesterase, nitric oxide continues to trigger the cascade, and blood flow into the penis is maintained. An understanding of this cascade led to the development and subsequent FDA approval of sildenafil in 1998 and tadalafil in 2003.

Comparing oral ED medications

Once a health care provider has determined that an oral ED medication is the best option for treating ED. The next step is comparing the risks and benefits of sildenafil and tadalafil to determine which is the better option for your circumstances. The success rate for these medications depends on the underlying condition but can range from 50% to 85% (Hsieh & Kuo, 2012).

The major difference between the two medications is the dosing options for tadalafil. Tadalafil can be prescribed daily or as needed prior to sexual activity. Tadalafil is also not affected by diet, whereas sildenafil is.

The following information is not meant to be complete or comprehensive. The list of side effects is only the most commonly noted in clinical trials and reported by the manufacturers. The list of drug interactions is incomplete and beyond the scope of this overview.

Comparison between two oral ED medications

Oral ED Medication: CharacteristicSildenafilTadalafil
Dosage25, 50, 100 mg tablets2.5, 5 for daily dosage 10, 20 mg tablets for on-demand
DosingAs needed, approximately 30 to 60 minutes before sexual activity, time to peak level 60 minutes, no more than once in 24 hoursAs needed, approximately 30 to 60 minutes before sexual activity or daily at the same time each day, time to peak level 120 minutes, no more than once in 24 hours
Time interval before sexual activity30-60 minutes before sexual activity, but it can be taken up to 4 hours before sexual activity60-120 minutes before sexual activity
Length of drug action4-5 hours36 hours
Dietary guidelinesA high-fat diet slows absorption1 Grapefruit and grapefruit juice may cause an interaction as can some herbal products such as St. John’s wortNot affected by food. Grapefruit and grapefruit juice may cause an interaction as can some herbal products such as St. John’s wort
ContraindicationsDo not take with nitrates, guanylate cyclase stimulators such as riociguat, or other PDE 5 inhibitors. Do not take with nitrates, alpha- blockers, guanylate cyclase stimulators such as riociguat, or other PDE 5 inhibitors. Do not use if you have non-arteritic anterior ischaemic
optic neuropathy.
Side effectsHeadache (15.8%), facial flushing (10.5%), upset stomach (6.5%), nasal congestion (4.2%), flu syndrome (3.3%), urinary tract infection (3.1%), abnormal vision (2.7%), diarrhea (2.6%), dizziness(2.2%), rash (2.2%), back pain (2.2%) and joint aches (2.0), other side effects reported less than 2%Headache (3%), upset stomach (4%), nasal congestion (4%), cough (4%), back pain (3%), upper respiratory infection (3%), muscle aches (2%), urinary tract infection (2%), Acid reflux (2%), abdominal pain (2%), other side effects reported less than 2% and dependent on dosage
Medication RisksProlonged erection (priapism)Prolonged erection (priapism)
Populations for whom the medication is not indicatedWomen and childrenWomen and children
Lifestyle precautionsSildenafil may cause dizziness and altered vision. See how the medication affects you before driving or operating heavy equipment. Alcohol can lower blood pressure, worsening symptoms. Tadalafil may cause dizziness and altered vision. See how the medication affects you before driving or operating heavy equipment. Alcohol can lower blood pressure, worsening symptoms.
Populations which may need a dosage adjustmentOver the age of 65 (40%), hepatic (liver) impairment, severe renal (kidney) impairment Hepatic (liver) impairment, renal (kidney) impairment
Sources: Pfizer product monograph and Eli Lilly Product Monograph

1 When sildenafil is taken with a high-fat meal, the rate of absorption was decreased by 29%, and the time it took to work was delayed (Pfizer, 2020).

All medications have potential side effects. These side effects can vary by individual. Before taking medications for erectile dysfunction, all men with cardiovascular disease should ensure that they are considered healthy enough for sexual activity.

The symptoms of an allergic reaction to medications may include itching, hives, swelling of the face or lips, difficulty breathing, and fainting. Allergic reactions require emergency care. This list is not meant to be comprehensive. If in doubt seek medical attention.

As an informed consumer, it can be helpful not only to know about drug information from a manufacturer’s perspective but also from a consumer. For that reason, Invigor Medical has collected reviews from consumers to help you further evaluate whether Sildenafil is a good choice for you.

Generic oral ED medications

When a medication’s patent expires, generic versions of the medication are usually produced. A patent is in place to prevent competition so that pharmaceutical companies can recoup some of the invested costs of testing and developing new medications. Manufacturers of generic medications do not have the same level of these costs and therefore can frequently offer the medications at a lower price. The active ingredients in generics medications such as sildenafil are the same as the brand name Viagra, as is tadalafil the same as Cialis.

Buying oral ED medications online

Both tadalafil and sildenafil require a prescription. If misused or prescribed incorrectly, serious side effects may result from taking any medication. Online companies that do not require a prescription are not legally dispensing medications. Telemedicine providers, such as Invigor Medical, both offer and require an appointment with a licensed health care provider to evaluate your medical history and determine whether you are a good candidate for an oral ED medication.

Another consideration is the pharmacy at which the prescription will be filled. You can verify the licensing status of a pharmacy with the relevant state pharmacy board. Invigor Medical uses the Olympia Compounding Pharmacy in Florida, whose license (#PH27414) can be verified through the Florida Department of Health.


While we strive to always provide accurate, current, and safe advice in all of our articles and guides, it’s important to stress that they are no substitute for medical advice from a doctor or healthcare provider.  You should always consult a practicing professional who can diagnose your specific case.  The content we’ve included in this guide is merely meant to be informational and does not constitute medical advice. 


  • Hsieh, J. & Kuo, Y. Erectile dysfunction: Causes, risk factors, and management. In Grant, P.S. (2012). Erectile Dysfunction: Causes, Risk Factors, and Management. Nova Science Publishers, Inc.
  • NIH Consensus Development Panel on Impotence. JAMA 1993 Jul 7; 270 (1):83-90
  • Pfizer Products, Upjohn Canada. 92020). Product Monograph, Viagra. Retrieved from
  • Lyseng-Williamson, K. A., & Wagstaff, A. J. (2002). Management of Erectile Dysfunction: Defining the Role of Sildenafil. Disease Management & Health Outcomes, 10(7), 431–452.

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