The popular drug Viagra can help men with erectile dysfunction to enjoy a satisfactory sex life. However, did you know that this drug might interfere with your body’s normal response to blood clotting?
As a result of its effect on blood clotting, Viagra is often classified as an anticoagulant, or blood thinner. However, does this mean that you should avoid taking it if you’re already on blood thinners, such as Warfarin or aspirin?
Let’s examine the evidence and find out if Viagra is an anticoagulant or blood thinner.
The Merck Veterinary Manual
To begin with, let’s look at what the Merck Veterinary Manual has to say about Viagra:
“Like other PDE-5 inhibitors, sildenafil citrate (Viagra) inhibits the action of phosphodiesterase type 5 (PDE-5), an enzyme that degrades cGMP. In experimental animals, sildenafil citrate has been shown to significantly increase the duration of erections and to improve male sexual behavior.”
Here we have it, straight from the horses’ mouths (or dogs’ mouths, in this case). Viagra increases the duration of erections in animals, which in turn can help to increase the animals’ sexual performance. Therefore, it makes sense that humans would also experience benefits from taking Viagra.
The Merck Index
If you’re a regular reader of my blog, you’ll know that I often like to consult reliable reference sources to find out the answer to medical questions. In this case, we’ll use the Merck Index, one of the most reliable and comprehensive sources of information on medicine. Here’s what they say about Viagra:
Viagra is also known as ‘sildenafil citrate’ and ‘sildenafil.’
“VIAGRA, an oral treatment for erectile dysfunction approved in more than 30 countries, inhibits an enzyme (phosphodiesterase type 5, or PDE-5) that breaks down the important chemical compound cGMP. Because cGMP causes blood vessels to relax, allowing blood to flow into erectile tissue, inhibiting PDE-5 can improve erection quality and duration. VIAGRA is not approved for use in children and is only available in select countries.”
So, as you can see, the Merck Index agrees with the Merck Veterinary Manual, stating that Viagra inhibits PDE-5, an enzyme that breaks down cGMP. cGMP stands for cyclic guanosine monophosphate and is a nucleotide, or “neurotransmitter.” Remember the transporters in our last blog? cGMP is one of the main compounds that transporters regulate.
Inhibition of PDE-5 by Viagra leads to an increase in the levels of cGMP in blood. This is important because cGMP plays a critical role in the body’s response to injury. When blood vessels are relaxed, due to reduced levels of cGMP, there is an increase in blood flow to injured tissue, which in turn can lead to faster recovery and less pain. Therefore, while Viagra may not be specific to reduce blood clotting, it does increase the body’s response to injury via an increase in cGMP levels.
Routine Coagulation Testing
Just in case you’re wondering, here’s a quick primer on coagulation:
Blood coagulation is the process by which blood forms clots, or agglomerates, to stop blood loss from either a blood vessel injury or an incision during surgery. Without blood coagulation, we would be susceptible to significant, and often fatal, blood loss.
In the U.S., the blood coagulation cascade is divided into four phases:
- Initiation
- Coagulation
- Fibrinolysis
- Clot lysis
The initiation phase begins when blood vessels are damaged, either by a needle or an incision. This damage causes the release of certain compounds, such as tissue factor (or thromboplastin) and high-mobility group box protein 1 (HMGB1). Tissue factor and HMGB1 are two compounds that trigger the coagulation cascade.
Once the coagulation cascade is initiated, blood begins coagulating at the site of injury. The protein C system is then activated, and forms a key role in the blood coagulation cascade. The protein C system is a serine protease that regulates the blood coagulation cascade. One of the functions of protein C is to inactivate two blood coagulation inhibitors, namely thromboplastin inhibitor and tissue factor pathway inhibitor (or ‘TFPI’)
Once activated, protein C cleaves and inactivates these inhibitors, allowing the coagulation cascade to develop fully. Furthermore, protein C has other important functions in the coagulation cascade, including the ability to activate protein S, which in turn activates pro-thrombin to form the prothrombinase complex. This complex inactivates the coagulation cascade by converting prothrombin into thrombin, which digests fibrin that accumulates during the clotting process. Thrombin also has the ability to convert fibrinogen into fibrin, resulting in the formation of blood clots. Therefore, due to its important role in blood coagulation, protein C is itself classified as a blood clotting enzyme.
In addition to its important role in blood clotting, protein C has additional important functions. For example, activated protein C can also down-regulate the production of pro-inflammatory cytokines, such as tumor necrosis factor alpha (TNF-α). Therefore, having too much TNF-α can lead to a hyper-inflammatory state, which in turn can lead to further complications.
The Role Of Viagra In The Blood Coagulation Cascade
Due to its effect on the blood coagulation cascade, Viagra can play an important role in stopping blood loss following an injury or surgery. This is why it is often prescribed, or suggested, for people who have had a heart attack, stroke, or pulmonary embolism.
However, does this mean that you should avoid taking it if you’re already on blood thinners, such as Warfarin or aspirin?
Remember, we’re scientists here at Life in Science, so we like to approach questions from a scientific point of view.
Here’s what the evidence says:
Can Viagra Increase The Bleed Threshold?
If you’re wondering whether or not Viagra increases the bleed threshold, it depends on how much you take. If you take too much, it can increase the risk of significant bleeding. This is why it is often suggested that people with renal dysfunction or those taking anticoagulants should avoid taking Viagra.
The following table shows the dose-related risk of bleeding with varying degrees of renal dysfunction. The results are summarized from a study conducted by Cardiovascular Research Technologies:
(Dose-related Risk of Major Bleeding With Varying Degrees of Renal Dysfunction)