Does Viagra thin the blood? No — Viagra does not act as a blood thinner. Its active ingredient, sildenafil, helps the blood vessels in the penis relax during sexual arousal; it does not stop blood from clotting. In the studies on its label it had no effect on bleeding time, did not add to the effect of aspirin and showed no significant interaction with warfarin. The exception is a man with a bleeding disorder or an active stomach ulcer: sildenafil has not been tested in him, so his doctor weighs it up first.
What is the difference between Viagra and a blood thinner?
They act on different things. Blood thinners help stop clots forming: anticoagulants such as warfarin, dabigatran, apixaban and rivaroxaban work by "stopping the production of clotting factors", and antiplatelet medicines such as aspirin and clopidogrel "stop small blood cells (called platelets) from sticking together and making a blood clot" (healthdirect — Blood thinners). Erection medicines "work by helping to relax the blood vessels in the penis", and they do not cause an erection without sexual stimulation (healthdirect — Erectile dysfunction). Sildenafil also lowers blood pressure a little and briefly — after 100 mg, by 8.4/5.5 mmHg on average at its lowest (EMA — Viagra). That matters for men on blood pressure medications; it has nothing to do with clotting.
Does Viagra affect platelets or bleeding time?
Not in the studies on its label. "VIAGRA had no effect on bleeding time, including during co-administration with aspirin," says the Australian product information. Platelets do contain the enzyme sildenafil blocks, but "Sildenafil, in limited studies, did not affect platelet function in vivo" (Viagra PI (Australia)). The one effect on platelets was seen in the laboratory: in test-tube studies, sildenafil strengthened the anti-clotting effect of sodium nitroprusside (EMA — Viagra) — a nitric oxide medicine that must never be used with Viagra in the first place (Viagra PI).
Can you take Viagra with a blood thinner?
Usually, yes: its label found no interaction with warfarin or aspirin. "No significant interactions were shown when sildenafil 50 mg was co-administered with tolbutamide (250 mg) or warfarin (40 mg)", and sildenafil 50 mg "did not potentiate the increase in bleeding time caused by aspirin (150 mg)" (Viagra PI). Tadalafil did not add to aspirin's effect on bleeding time either; with warfarin it lowered warfarin levels by 11% and 13% without changing its effect on clotting time, and the Australian label asks doctors to keep in mind that the INR may rise or fall when a man starts or stops tadalafil (Cialis PI (Australia)). The detail, and the newer blood thinners the labels do not name, are in can you take Viagra while on blood thinners or warfarin? and can aspirin be taken with Viagra?
Who needs more care before taking Viagra?
Men with a bleeding disorder or an active stomach ulcer. There is no safety information on sildenafil in these men, so the Australian product information says it "should be administered with caution" (Viagra PI), and the European label says only "after careful benefit-risk assessment" (EMA — Viagra). The Viagra leaflet asks you to tell your doctor before you start if you have "any bleeding disorder such as haemophilia" or a stomach ulcer (Viagra CMI).
Can Viagra cause bleeding?
A few kinds of bleeding are listed as side effects, none of them common. Nosebleeds and blood in the urine are uncommon, in up to 1 man in 100; bleeding in the retina, from the penis or into the semen is rare, in up to 1 in 1,000 (EMA — Viagra). The Australian leaflet says to call your doctor straight away about bleeding from the nose or blood in the urine (Viagra CMI). Bleeding piles are not on the list — see can Viagra make haemorrhoids worse or cause bleeding?
Can blood thinners cause erection problems?
They are not listed as a cause; the heart and blood-vessel conditions they are prescribed for often are. That question is answered in can blood thinners cause or help with erectile dysfunction?
The short answer
Viagra is not a blood thinner. In the studies on its label it did not change bleeding time, with or without aspirin, and did not interact with warfarin, so it can neither replace a blood thinner nor, in those studies, add to one. Men with a bleeding disorder or an active stomach ulcer need their doctor's assessment first, and a nosebleed or blood in the urine after a dose is a reason to call the doctor. More situations like this are covered in our guide to Viagra and Cialis with special conditions.
