Does the copper coil cause weight gain?

What the research says

The copper coil is hormone-free. It contains no oestrogen, no progestogen and no other systemic medication. It works by releasing copper ions into the womb, which create an environment that is hostile to sperm. Because it leaves your hormonal system alone, there is no biological mechanism by which it could directly cause weight gain.

Clinical studies back this up. The copper IUD is often used as the hormone-free comparison group in research on contraception and weight, including the Cochrane review "Progestin-only contraceptives: effects on weight" (Lopez and colleagues, 2016). Those studies use it as the baseline precisely because it has no hormonal effect, and they have not found it to cause weight gain.

Hormonal methods vary. The contraceptive injection has the strongest evidence of weight gain, and the implant has been linked to modest changes in some women. For the Mirena coil, controlled studies have not found a significant difference in weight gain between users and non-users. The copper coil has no hormonal effect on weight at all.

So why do some women gain weight after getting a copper coil?

Because weight gain happens for lots of reasons, and the timing is often coincidental.

Women tend to have a copper coil fitted at particular life stages. It might be after having a baby, when coming off the pill, or in their late twenties and thirties, as metabolic rate naturally begins to slow. Weight often changes at all of these points, whatever contraception you use.

Coming off the combined pill is a particularly common confounder. The combined pill suppresses androgens and can reduce appetite and fluid retention in some women. When you stop it and switch to a hormone-free method like the copper coil, your body returns to its baseline hormonal state. If the pill was masking a tendency to gain weight, the gain can seem to arrive with the coil when it is really about stopping the pill.

There is also the psychological side. If you are worried about weight gain as a side effect, you are more likely to notice small fluctuations and put them down to the coil. A kilogram of normal daily variation (fluid, food, bowel contents) can feel like confirmation of a fear.

Copper coil side effects that are real

The copper coil does have side effects. Weight gain just isn't one of the evidence-based ones.

The main side effects are heavier, longer periods and more menstrual cramping, particularly in the first 3-6 months after fitting. For most women this settles. For a minority, periods stay heavier than they were before the coil, and this becomes a reason for removal.

Some women also report more PMS-like symptoms (bloating, breast tenderness, mood changes) around their period. It is debated whether the coil causes this directly, or whether heavier periods simply make existing PMS more noticeable. There is no established hormonal mechanism, since the coil is hormone-free. What patients describe is still real, and we take it seriously.

What to do if you have gained weight with a copper coil

The coil is unlikely to be the cause, but the weight gain is still worth looking into.

A well woman check or blood test panel can rule out thyroid dysfunction, insulin resistance and hormonal imbalances that might explain the change. These are common, often undiagnosed conditions that can cause gradual weight gain, and they are easier to manage when caught early.

A broader hormonal assessment is particularly worth doing if the weight gain comes with other symptoms: fatigue, hair thinning, mood changes, or irregular periods despite the coil.

Come and see us if you are unhappy with the copper coil for any reason, weight-related or otherwise. You can have it removed at any time, and removal is £400 all-inclusive for the consultation and the removal. A gynaecologist can talk you through the options: a hormonal coil like the Mirena or the Kyleena, or a different contraceptive method altogether, may suit you better.

The bottom line

The copper coil does not cause weight gain. There is no hormonal pathway for it to do so, and controlled studies have not found an association. If you have gained weight since your coil was fitted, the cause is almost certainly elsewhere. It is worth finding out what it is before you remove a contraceptive that is otherwise working well.

If weight is your main worry about the copper coil, you can set it aside. Its hormone-free status makes it one of the least likely contraceptives to affect your weight.