Priapism is caused by a problem with the blood supply to the penis. Certain medical conditions and medications can prevent blood leaving the penis.


When a man becomes sexually aroused, his nervous system releases a chemical called nitric oxide. Nitric oxide relaxes and widens the walls of the arteries that supply blood to the penis. This increases the blood flow to the spongy tissue of the penis, causing it to expand and harden into an erection.

After the feelings of sexual arousal have passed, the arteries in the penis should contract (become narrower), which pushes the excess blood out of the penis and returns it to its normal floppy state.

Anything that affects the nervous system or blood flow (or both) can trigger priapism.

Sickle cell anaemia

Sickle cell anaemia is an inherited blood condition where red blood cells develop abnormally. Red blood cells are usually round and flexible and carry oxygen from the lungs to the rest of your body.

However, in people with sickle cell anaemia, the shape and texture of red blood cells can change. They become hard, sticky and crescent-shaped.

In boys and men, it is possible for the hardened blood cells to clump together. This can lead to slow blood flow and clotting in the blood vessels of the penis, which can cause a painful and persistent erection.


A number of medications can sometimes disrupt the normal workings of the nerves that help trigger an erection by widening the arteries in the penis.

The nerves essentially ‘forget’ to narrow the arteries after the feelings of sexual arousal have passed, leading to ischaemic priapism.

Medication associated with ischaemic priapism includes:

  • medications for erectile dysfunction that are injected directly into the penis, such as alprostadil
  • blood-thinning medications, such as warfarin and heparin
  • some types of antidepressants, such as fluoxetine (Prozac), bupropion, and antipsychotic agents
  • some medications used to treat high blood pressure (hypertension), such as calcium channel blockers

In addition, some recreational drugs have also been linked to priapism, including:

  • cocaine
  • ecstasy
  • methamphetamine (crystal meth)
  • cannabis

Less common causes

Less common causes of priapism include:

  • thalassaemia – a blood condition similar to sickle cell anaemia
  • cancers of the blood, such as chronic leukaemia and multiple myeloma
  • cancers that have spread from nearby tissue or organs, such as prostate cancer or bladder cancer, and are disrupting the flow of blood inside the penis
  • spinal cord injury
  • blood clots 
  • Fabry disease – a rare, genetic condition that affects the metabolism (the process that converts food into energy)  
Page last reviewed: 10/02/2015
Next review due: 01/02/2018