Hormonal contraception and blood clots
You might've read that the combined pill can increase your risk of thrombosis. But what does this really mean? And what are the risks? We explain all you need to know about contraception and blood clots.
What are blood clots?
Your body creates blood clots when you have a cut or wound. They help stop you from bleeding too much and start the wound healing. For a blood clot to form, parts of your blood start to thicken (coagulate) and become more solid. When your injury heals, this blood clot dissolves back into your body.
You might also see blood clots in your period. These appear like fleshy bits of blood you might find on a tampon or sanitary towel. They are usually nothing to worry about, although this may be a sign of a heavy period.
Sometimes a blood clot forms when there’s no injury. When a clot appears where it’s not supposed to – such as in a vein taking blood from your leg back to your heart – it’s called a deep vein thrombosis (DVT).
What is a deep vein thrombosis (DVT)?
Deep vein thrombosis, or DVT, is when a blood clot forms in a vein. This blocks the vein so that blood builds up behind the clot.
The most common area for DVT is in the veins of the calf. It can cause pain and swelling.
If you have symptoms of DVT, such as pain and swelling in the leg, ask for a GP appointment urgently. If you have pain and swelling with breathlessness and chest pain, call 999 or go to A&E urgently.
If any of these symptoms start when you’re taking combined contraception, stop taking the contraception and speak to your GP or local sexual health clinic immediately for medical advice. Remember to take steps to stop unplanned pregnancy if you stop your usual contraception method.
What does a DVT feel like?
The main symptoms of a DVT in the leg:
pain
swelling
redness
warmth
This is usually on one leg – it can be helpful to compare one leg to the other.
DVT can lead to pulmonary embolism
Once a blood clot has formed, a piece of the clot can break off and travel back through the heart to the lungs, where it can cause a further blockage in the small veins of the lungs.
This is called a pulmonary embolism. It’s a life-threatening condition that needs urgent treatment.
The main symptoms of a pulmonary embolism are chest pain and shortness of breath.
Are you at risk of DVT?
The combined pill, and other combined methods of contraception like the patch and the ring, slightly increase the tendency of your blood to clot. So they slightly increase the chance of getting a thrombosis.
This means that it’s important to consider whether you have an higher chance of developing DVT before choosing these methods.
You have a higher risk if you:
have reduced mobility (for example, if you have difficulty walking)
smoke
have a BMI (body mass index) over 35
have a blood disorder that increases the tendency of your blood to clot
have had DVT before
have varicose veins
Before anyone prescribes you a combined hormonal contraceptive, they’ll ask you about these.
It’s also worth bearing in mind that long periods without any movement – such as long flights or recovery after surgery – also increase your risk of thrombosis. High altitudes can also increase the risk.
How high is the risk of thrombosis with the pill?
The risk of DVT in the leg or lung of a healthy young person is low. About 2 in 10,000 people a year experience this.
The pill, along with other combined contraceptive methods such as the patch and the vaginal ring, can increase your risk of a deep vein thrombosis:
risk of DVT among people taking the pill: 6-12 in 10,000
risk of DVT among people not on the pill: 2 in 10,000
risk of DVT during pregnancy: 7-25 in 10,000 pregnancies
Of those who get DVT on the pill (6-12 people in 10,000), approximately 1% of cases are fatal.
The risk is highest after you first start taking it
The risk of DVT is highest after you first start taking the pill, or start again after a break of a month or more. Then the risk goes down over a year and stays at the same level after that. For this reason, repeatedly stopping and starting the pill is not recommended.
Not all pills carry the same risk
Different combined pills carry a different risk of DVT. It depends on the type of synthetic progesterone they contain:
Combined pills that contain levonorgestrel or norethisterone have a lower risk.
Combined pills that contain desogestrel, drospirenone’ or gestodene have a higher risk.
Learn more about types of combined pill
What about progestogen only pills?
Pills containing progestogen only, known as the POP or mini-pill, do not increase your risk of blood clots. This includes those that only contain desogestrel.
Learn more about the progestogen only pill
