Don’t panic! Six common causes of irregular bleeding on the pill

Two of the most common forms of contraception are the combined pill (known simply as “the pill”), and the progesterone only pill (known as the “mini pill”). The pill contains both oestrogen and progestin, a synthetic version of progesterone, to prevent ovulation, while the mini pill only contains progestin.

These hormones suppress a part of the brain known as the pituitary gland, which stops sex hormones being released, in turn preventing ovulation.

The pill is used for contraception, to regulate periods, to reduce period pain, to help with premenstrual symptoms, or any combination of these. The pill is not suitable for everyone, and some are better off using the mini pill.

Differences in bleeding between the combined pill and the mini pill

With the combined pill you generally take it for three weeks, then have one week where you have a “pill break”, during which bleeding will occur. This happens because the body has a break from synthetic oestrogen, and the womb lining which has built up with the oestrogen stimulation over the previous weeks then breaks down with the sudden drop in oestrogen levels.

It is completely normal for this to happen with the combined pill. This has been a traditional way of the taking the contraceptive pill because it mimics the normal menstrual cycle with a “period” each month. New preparations of the combined pill have extended hormone dosing so that people can get periods less than once a month.

The mini pill is taken daily with no break, and with some older mini pills it needs to be taken strictly within a three hour window each day. As there is no pill break, there is no set withdrawal bleed. But people can respond to the extra progestin in different ways — and bleeding can become irregular or unpredictable.¹

Those who take the mini pill are more likely to experience irregular bleeding compared with those who take the combined pill. The irregular bleeding is generally nothing to worry about, and does not mean that the pill is not working

So, those who take the mini pill are more likely to experience irregular bleeding compared with those who take the combined pill. The irregular bleeding is generally nothing to worry about, and does not mean that the pill is not working. But if you find the bleeding too troublesome, speak to your GP as an alternative contraception may be more appropriate.

Although it is more common to have irregular bleeding with the mini pill, bleeding can occur with both types of pills, and there are many reasons why this is the case. Some of these are detailed below.

When starting the combined pill

It is common to have irregular bleeding within the first three months of starting the combined pill. This is because the body is adjusting to the increased amount of oestrogen and progesterone. Each person’s womb reacts differently to this initial fluctuation in hormone levels. In some people, the womb lining thickens excessively, which means there would be shedding of the lining when there are temporary dips in progesterone and oestrogen levels, such as before the next tablet is taken the next day.

Progesterone only pill causing irregular bleeding

Mini pills have a variable effect on periods — some people continue to have regular periods (up to 50%), some experience bleeding/spotting episodes that last longer than 14 days (20%) and some have no periods or infrequent bleeding (up to 50%).² Some people just get spotting. This is because the womb lining of different people responds differently to the mini pill, and it is unpredictable.³

Breakthrough bleeding when the combined pill is taken back-to-back

Some people try to avoid getting a period on the combined pill by omitting the seven day pill break, and taking the combined pill continuously for two months. In this situation, there is continuous oestrogen exposure, which stimulates the endometrium and keeps the lining thick. When it becomes so thick, it also becomes fragile and is more likely to bleed. So, many people find they are more likely to experience spotting when this happens. Taking contraceptive pills back-to-back won’t affect how they work as a contraceptive, and recent guidelines note that some pills licensed to be taken for 21 days followed by a seven day break are safe for continuous use.⁴ Be sure to consult with your doctor before making any changes to your regime.

Forgetting to take the pill

If you accidentally forget to take the combined pill there is a sudden drop in the oestrogen level which stimulates the womb lining to shed, causing a withdrawal bleed. This may not happen to everyone. When the pill is missed on on one day, you are still protected from pregnancy, and you can just take the missed pill as soon as possible, even if this means taking two pills on the same day.⁵ Then continue to take the rest of the pack as normal.

However, if the pill is missed on two or more days then you may not be protected against pregnancy. Similar to missing one pill, just take the last pill that is missed or follow the guidelines on the pack, and continue with the rest of the pack as normal. You will need to use extra protection such as condoms for the following seven days to ensure that you are protected against pregnancy.⁶

Accidentally taking two pills in one day

Taking two pills means that there will be excess oestrogen, which stimulates the womb to have a thicker lining. Because of this, on the following day when the pill is taken there is a relative drop in oestrogen levels, and bleeding may occur due to shedding of the womb lining. However, this does not necessarily happen to everyone.

Secondary health conditions

There may be secondary health reasons for irregular bleeding while on the pill, unrelated to the pill itself. For example, the pill is about 91% effective for typical use, which refers to how the pill is typically used in real life. If taken correctly (“perfect use”), it is more than 99% effective,⁷ which means you can still get pregnant on it, especially if this is the result of missing two or more pills in a row. If pregnancy occurs, there is a small chance it may be an ectopic pregnancy, where the pregnancy develops outside the womb. This can present with vaginal bleeding and severe pelvic pain.

Bleeding disorders such as von Willebrand’s disease, an inherited condition where there is a lack of a clotting factor, can lead to irregular bleeding while taking the pill. The menstrual bleeds with this condition may be heavy, and there may be other symptoms such as easy bruising and excessive bleeding after dental extraction.

As a general rule, if you feel the menstrual bleeding you are experiencing while being on the pill is unusual, frequent, or heavy, then you should see a GP

Chlamydia and sexually transmitted infections may also cause irregular bleeding. This may also be associated with pain on passing urine, abnormal vaginal discharge which is yellow or milky white, and lower abdominal pain.

Endometrial or cervical polyps, which are benign outgrowths of the womb lining, can cause irregular bleeding due to the polyp being fragile and bleeding. These may not cause any other symptoms other than bleeding.

Cervical and endometrial cancers can also present with irregular bleeding. There may be few other symptoms, but there may be vaginal discharge or vaginal discomfort. It is important, therefore, to have regular smear screening to allow for early detection.

As a general rule, if you feel the menstrual bleeding you are experiencing while being on the pill is unusual, frequent, or heavy, then you should see a GP as further tests may be needed.

Featured image shows a menstrual cup on its side on a pink background. Some liquid that looks like blood is inside it and spilling out of it.

Last updated May 2023
Next update due 2026