Remedies for Erectile Dysfunction - Viagra, Levitra and Cialis

For a long time treatment for impotence was restricted to penis injections, mechanisms that create voids or prostheses implanted through surgery. Since late 1990s, however, there has been a simple and convenient treatment for erectile dysfunction, drugs that inhibit Phosphodiesterase type 5 (PDE-5), commercially known as Viagra, Cialis and Levitra.

In this paper we explain how these drugs work, their indications and side effects.


To understand how the PDE-5 works, you must understand how a normal erection occurs.

There are two widely vascularized cylindrical structures inside the penis, called corpora cavernosa. When sexual stimulation occurs, blood flow to these structures increases, causing increased penis size and consequent compression of the penile veins. These are veins that are responsible for returning blood to the rest of the body and therefore, when they are compressed, they are unable to perform its role. This leads to blood retention in the corpora cavernosa, maintaining an erection, which is only interrupted when sexual stimulation ends. When there is an orgasm or when the stimulus for the erection ends, the corpora cavernosa and the penis becomes empty and flaccid again.

When the corpora cavernosa is filled with blood it takes a signal from the central nervous system, releasing a substance called nitric oxide. This neurotransmitter relaxes the blood vessels of the cavernous body, helping the entry of blood in it. Until there is nitric oxide in the corpora cavernosa, the penis will remain engorged with blood, and therefore, erect, when nitric oxide level falls, the erection ends.

The erection, however, is a more complex process than that explained above. Hormonal and psychological factors interfere with this mechanism. For example, men with low testosterone level cannot produce sufficient amounts of nitric oxide. The same situation can occur during periods of stress or anxiety. Erection therefore depends on vascular, neurological, hormonal and psychological factors. A patient with diabetes and diseased vessels may be unable to supply enough blood for the penis to fill, even if there is enough nitric oxide.


Not to go into details about complex biological and chemical mechanisms, we can say roughly that the substance responsible for the emission of nitric oxide action, which is responsible for the erection, is called Phosphodiesterase type 5 (PDE-5). There are drugs that inhibit PDE-5, which have a more prolonged action of nitric oxide, thus ensuring ease of the blood flow to the corpora cavernosa.

There are currently three different drugs that act on the inhibition of PDE-5:

• Sildenafil (Viagra)
• Vardenafil (Levitra)
• Tadalafil (Cialis)

Knowing now how erection works and how these drugs act, it is possible to imagine that Viagra or other PDE-5 inhibitors may not have an effect on some patients, especially on those with problems with the nerves and vessels of the penis. The success rate is around 70%, being higher only for patients with psychological problems and lower for those with poorly controlled diabetes or pelvic nerve damage after prostate surgery.


Sildenafil, marketed under the name Viagra since 1998, was the first drug in the class of PDE-5 inhibitors to be launched. It is currently the oral drug for impotence most tested and studied.

Viagra is sold in tablets of 25 mg, 50mg or 100mg, with a dosage ranging from 25mg to 100mg per day, depending on the clinical evaluation. The tablet should ideally be taken on an empty stomach (at least 2 hours after the last meal) and 1 hour before an intercourse. Sildenafil is not a medicament for everyday use, the minimum interval between two dosages should be at least 24 hours.

The action of Viagra starts after 30 minutes and lasts for up to four hours, but that does not mean that erection will continue during all this time. Sildenafil only helps erection to occur because it increases the lifetime of nitric oxide already produced, not directly affecting its production. If there is sexual stimulation, there is no stimulus for the release of nitric oxide and erection, just as it typically occurs.

Contraindications of Viagra

Viagra is a very safe drug, with more than 12 years on the market. However, like any medicine, there are contraindications and side effects. It is contraindicated for patients who take nitrates to use Viagra.

Sildenafil is a vasodilator and although preferably acts on the vessels in the penis, there is also an increase in nitric oxide causing a reduction in systemic blood pressure. In healthy people this drop is small and is usually asymptomatic. However, the use of Viagra with drugs for hypertension should be made carefully. The nitrate (Isordil, monocordil, Monoket etc ...), used for heart disease, are vasodilators whose action is potentiated by sildenafil excessively, may cause a significant drop in blood pressure, syncope and even cardiac.

Drugs used for prostate hypertrophy, terazosin and doxazosin, also have vasodilator and are also enhanced by the Viagra, which may cause hypotension. In this class, tamsulosin appears to be a safer drug to be taken along with Viagra.

Patients with heart failure, Viagra should not be used without explicit medical clearance, as it may cause decompensation of the frame. Patients who had a stroke, especially in the last six months, must also be careful with the use of sildenafil.

Some drugs alter the effect of Viagra and its consumption should also be avoided, including: Erythromycin, ketoconazole, cimetidine, itraconazole, rifampicin, phenytoin, indinavir and ritonavir.
Viagra should not be used in combination with other PDE-5 as Cialis and Levitra. Erectile improvement with this combination is highly possible but there is still a high risk of toxicity. The PDE-5 inhibitors in very high doses (above mentioned) can cause a stroke and a serious hypotension.

Side effects of Viagra

The most common side effects of sildenafil are headaches and dizziness caused by the effects of systemic vasodilation. Heartburn is also a common symptom.

There are other effects, which are rare, ant they include visual disturbances such as bluish vision, blurred or increased sensitivity to light. Hearing disorders and hearing loss (sometimes permanent) may also occur.

Priapism, which is a prolonged, painful erection, is a very rare event, but may occur after use of Viagra. Every patient with continuous erection for more than four hours should call the emergency because there is risk of ischemia and thrombosis of the penis. Priapism is a common side effect when Viagra is used recreationally by young people combined with other drugs such as ecstasy, cocaine and amphetamines.

Levitra is a drug launched in 2003, sold in tablets 2.5mg, 5mg, 10mg and 20mg. As Viagra, it is to be taken an hour before intercourse and preferably away from the last meal. The action also lasts for about 4 hours.

The profiles of side effects and contraindications of Levitra are similar to Viagra.

Cialis was also released in 2003 and is sold in tablets of 5mg, 10mg and 20mg. There are no differences between the effectiveness of Cialis, Levitra and Viagra, but the former has some peculiarities: Cialis has a longer effect, which lasts for up to 36 hours, does not seem to cause the rare vision changes as Viagra and Levitra, and can be taken close to meals. Apart from these minor differences, the three drugs are very similar.