How common are blood clots on the contraceptive pill?

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The Feed asked experts to explain the risk of blood clots on the contraceptive pill and how it compares with the AstraZeneca vaccine.

Blood clots - although rare - are a risk associated with both the contraceptive pill and the AstraZeneca vaccine. 

Due to reports of blood clots in the UK and a possible case in Victoria, the government updated its health advice last Thursday, recommending those under 50 to get the Pfizer jab instead of the AstraZeneca vaccine.

Speculation over the safety of the AstraZeneca jab prompted comparisons online with the contraceptive pill.  This medication remains Australia’s default contraceptive. So what does that mean for those taking it?

Blood clots: The pill vs the AstraZeneca vaccine

The rate of blood clots in the AstraZeneca vaccine is roughly one in every 250,000.

The risk of getting a blood clot per year from being on the pill is one in 2,500.

However, the risk of developing a blood clot increases during pregnancy,  according to Jenny Doust, a practising GP and clinical professorial research fellow at the University of Queensland.

This means “if you’re taking the pill to avoid becoming pregnant then you’re reducing your risk of having a blood clot.”

While blood clots are more prevalent among those taking the pill, they are generally more severe when associated with the vaccine.

Professor Doust said the most common blood clot on the pill is deep vein thrombosis - a blood clot in your leg that is serious but has a very low risk of fatality.

And this particular type of blood clot differs from the blood clot associated with the AstraZeneca vaccine. 

There are other alternatives to taking the Pill
The risk of blood clots associated with the pill is rare.
PhotoAlto/Getty Images

Professor Doust said the blood clot linked to the vaccine generally occurs in the brain and carries a 25 per cent risk of death.

“They’re very different. I don’t think we should be comparing the two things at all,” Professor Doust told The Feed.

Blood clots in the pill relate to the increased ability of platelets to stick together, said Dr Geoff Mitchell, Emeritus Professor of General Practice at the University of Queensland.

“When there’s been a tear in a blood vessel, [platelets] aggregate and they start the process of plugging up that tear,” Dr Mitchell said.

Dr Mitchell said in rare cases, the AstraZeneca vaccine appears to reduce the number of platelets that are available. 

“The same time as creating clots, you’re also not replacing the platelets that cause the clots in the first place,” he told The Feed.

This rare occurrence results in a combination of bleeding and clotting because platelets, which make blood clots, are used up and unable to be replaced. 

Despite this, Dr Mitchell said the risk of blood clots in older people is much rarer than the prospect of getting really sick with COVID-19.

“For younger people, where the risk of getting really sick from COVID is a lot less, the burden of clotting is different and so you’d be more hesitant to prescribe the AstraZeneca vaccine,” he said.

Dr Mitchell said blood clotting with the AstraZeneca vaccine is so rare that it didn’t show up in preliminary tests.

“When using millions of doses, rare things are going to start to show up,” he said.

A nurse handles a vial of Covid-19 vaccine at the Camp Hill Medical Centre n Brisbane on Monday, March 22, 2021.
AAP/Dave Hunt

How much information is given to women about the risks?

Before being prescribed the contraceptive pill, women should be quizzed on their medical history and that of their family and whether there are any other risk factors for blood clots, Professor Doust said.

“If you have very high-risk factors for blood clots, you should probably be looking at alternative forms of contraception,” Professor Doust said.

Different pills have varying risks of blood clots but Professor Doust said most pills that women are prescribed in Australia are quite safe.

“It concerns me that women are perhaps being made anxious unnecessarily by the current information that’s being put out.”

Professor Doust said it’s important that all health considerations are checked out before women take the pill, particularly as they age and the risk of developing blood clots becomes greater.

“That’s why women should have to come back to the doctor to be prescribed the pill and not just be provided it over-the-counter,” she said.

“I don’t think women should be jumping into stopping the pill.”

There is less risk of death with blood clotting associated with the pill.

While researching this story, The Feed received a flood of emails and messages from young women about their experiences with the contraceptive pill.

Many young women claimed they were not warned about the risk of blood clots and had only been told about possible weight gain or its impact on acne. 

“I suffer with chronic migraines and I was on the pill for 20 years without once being warned of the clot risks,” one woman told The Feed.

“I recently went to a new neurologist and he was really upset that with my heightened risk for clots I’d never had the conversation with my GP.”

Others told The Feed they had more positive experiences and that their GPs recommended the best possible contraceptive for their individual medical needs.

“I have a strong family history of blood clots. I went to the doctor and he told me that they’d do a blood test to check to see if I was a carrier of the condition prior to me commencing,” one woman said.

“About 24 hours later, I was called and told to throw them all out. But I’m fortunate.”

Kath - not her real name - told The Feed she was prescribed a contraceptive pill when she was 21 to clear up her acne. But like many other women who reached out to The Feed, she said she was not told of any potential side effects. 

“I didn’t realise any of the side effects until I started speaking with my friends who mentioned weight gain or mood swings,” she said.

“I started to have a read online and saw other side effects mentioned including blood clots.”

A healthcare worker prepares a syringe with the Astra Zeneca Covid19 dose at Coria City Hospital.
Australia has changed its advice for the AstraZeneca vaccine.

Kath said when she renewed her prescription, there was no further conversation about the pill and its potential side effects.

She believes GPs should have a greater responsibility to highlight risks “even if there’s a slim chance” of developing them.

Jessica - who also requested for her name to be changed - told The Feed she was not provided in-depth information about the pill's potential side effects when she was prescribed it at 17.

“The doctors didn’t ask me about risks that might have been in my family like heart attack or stroke, and didn’t ask about blood clots in particular,” she said.

Jessica said she suffered mood swings on the pill but when she raised it with doctors, they dismissed her concerns.

She said she had to push her doctors to put her on different contraceptive pills on various occasions and has now decided to change tack when it comes to contraception.

“You’ve got to argue and argue or go back multiple times for them to understand that this isn’t just a one-off thing, I’m not being dramatic or trying to get attention. It’s a serious issue,” she told The Feed.

But Dr Mitchell was at pains to emphasise that people should not be concerned about the risk of blood clots when it comes to the pill or the vaccine.

“Every medicine runs the risk of side effects, probably you’re safer getting that vaccine than crossing the road.”

“If we want the infection to die out, we’ve got to have herd immunity. We’re [getting vaccines] for the good of the country as much as for the good of getting it for ourselves.”