What Is an IUD? How It Works, Types & Side Effects

An intrauterine device, or IUD, is a small, T-shaped device placed inside the uterus to prevent pregnancy. There are two broad categories: copper IUDs, which contain no hormones, and hormonal IUDs, which release a low dose of a progestin called levonorgestrel. Both are among the most effective contraceptives available, with failure rates well under one percent per year, and both work for years before needing replacement. The specifics of how they prevent pregnancy, what side effects to expect, and who they work best for differ meaningfully between the two types.

How Copper IUDs Work

A copper IUD prevents pregnancy primarily by creating a local inflammatory response inside the uterus. The copper ions released from the device are toxic to sperm, impairing their motility and viability so that fertilization becomes extremely unlikely.¹ There is no hormone involved at all. The device itself, typically wrapped in fine copper wire, begins working almost immediately after insertion, which is part of why copper IUDs can also serve as emergency contraception. The contraceptive effect is fully reversible: once the device is removed, the inflammatory environment resolves and fertility returns.

How Hormonal IUDs Work

Hormonal IUDs release levonorgestrel, a synthetic progestin, directly into the uterine cavity. This has two main effects: it thickens cervical mucus, making it difficult for sperm to reach an egg, and it suppresses the uterine lining, making the environment inhospitable to implantation.¹ Because the hormone is delivered locally rather than absorbed from a pill or implant, the amount that enters the bloodstream is much lower than with oral contraceptives or subdermal implants.² That low systemic exposure is often cited as the reason many people tolerate hormonal IUDs better than pills or the patch, though it does not mean systemic effects are zero.

Available Types

In most countries, the copper IUD is sold as the Paragard (or comparable brands) and is approved for up to ten years of use, though some evidence suggests it remains effective even longer. Hormonal IUDs come in several versions that differ mainly in how much levonorgestrel they release each day. A pharmacokinetic analysis of the three available hormonal IUDs confirmed that the 52-mg system (marketed as Mirena) delivers the highest dose, followed by the 19.5-mg system (Kyleena), with the 13.5-mg system (Skyla/Jaydess) delivering the lowest.² The higher-dose Mirena is approved for up to eight years. Kyleena is approved for five years, and Skyla for three. A newer 52-mg system, Liletta, has a similar profile to Mirena and is also approved for up to eight years.

Choosing among them often comes down to priorities. If you want to avoid hormones entirely, the copper IUD is your only IUD option. If lighter periods or managing heavy bleeding is a goal, a higher-dose hormonal IUD is usually more effective at reducing flow. Smaller-framed hormonal IUDs like Kyleena and Skyla were designed partly with younger and nulliparous users in mind, though current evidence supports all IUD types as safe regardless of whether someone has given birth.

How Effective Are IUDs

Both types of IUD are highly effective, but hormonal IUDs have a slight edge. A large European surveillance study tracked tens of thousands of users and found an overall Pearl Index of 0.06 for the levonorgestrel IUD and 0.52 for the copper IUD. Put simply, for every thousand people using a hormonal IUD for a year, fewer than one experienced a contraceptive failure, while roughly five out of a thousand copper IUD users did.³ Both numbers are far better than typical-use failure rates for pills, patches, or condoms. The key advantage of IUDs over those methods is that they require no daily or per-use action, so the gap between “perfect use” and “typical use” is essentially zero.

Side Effects of Hormonal IUDs

The most common side effect of a hormonal IUD is a change in bleeding patterns, and for most people, this means lighter periods over time. A systematic review found that very few users experienced amenorrhea (no period at all) in the first 90 days, but the rate climbed steadily: about 8% by six months and roughly 18% at some point during the first year.⁴ Another study found that people with lighter baseline periods were more likely to stop bleeding entirely, while those with heavy or prolonged flow before insertion were less likely to reach amenorrhea in the first six months.⁵

Irregular spotting is common in the first three to six months, which can be frustrating even though it usually resolves. Other reported side effects include acne, breast tenderness, headaches, and mood changes. The mood question is genuinely unresolved. A systematic review of studies on psychiatric symptoms and levonorgestrel IUDs found mixed results: ten studies reported increased depressive symptoms, two actually found reduced symptoms, four found no association, and others were uncertain.⁶ The honest summary is that some individuals clearly experience mood changes on a hormonal IUD, but the average effect across large populations is hard to pin down. If you have a history of depression or anxiety and notice worsening symptoms after insertion, it is worth discussing with your provider rather than assuming it is unrelated.

Side Effects of Copper IUDs

Copper IUDs tend to push bleeding in the opposite direction: heavier and longer periods, especially in the first several months. Research using objective measurements (weighing used menstrual products) has shown that copper IUDs increase menstrual blood loss by about 50% compared to pre-insertion levels, and this increase stays relatively constant over at least the first year.⁷ A separate study found that menstrual pain scores also increased significantly at three months after copper IUD insertion, and the duration of bleeding was prolonged.⁸

Interestingly, many copper IUD users report that their bleeding and cramping improve over time, but objective measurements do not always confirm that perception. The subjective improvement may reflect adaptation to the new normal rather than an actual reduction in blood loss. If your periods were already heavy or painful before getting a copper IUD, these side effects are more likely to be a real issue for you. For people with light or moderate periods who want hormone-free contraception, the increase is often manageable.

What Insertion Feels Like and How Pain Can Be Managed

Insertion involves passing the IUD through the cervix and into the uterus, which takes about a minute but can be quite uncomfortable. Pain levels vary widely from person to person. A randomized trial found that a paracervical block (a numbing injection around the cervix) reduced median pain scores during placement substantially compared to no block.⁹ The trade-off is that the block itself involves a needle and some discomfort. A network meta-analysis comparing multiple pain-relief strategies found that a lidocaine-prilocaine cream applied to the cervix ranked highest for reducing pain both during IUD insertion and at the moment the tenaculum (the instrument that steadies the cervix) is placed.¹⁰

If pain management during insertion is a concern for you, it is worth asking your provider about these options ahead of time. Some clinics now routinely offer numbing agents, while others still default to telling patients to take ibuprofen beforehand. You have every right to request better pain control.

Uterine Perforation

Uterine perforation, where the IUD punctures through the wall of the uterus, is the complication people worry about most, and it is genuinely rare. A large cohort study of over 326,000 IUD users found a cumulative incidence of about 0.2% at one year and 0.6% at five years.¹¹ The biggest risk factor is timing relative to childbirth. Having an IUD placed between four days and six weeks after delivery carried about seven times the risk of perforation compared to non-postpartum insertion.¹² Breastfeeding at the time of insertion also modestly increased the risk. A European surveillance study found a roughly sixfold increase in perforation risk among breastfeeding users, though the absolute numbers remained very small.¹³ The take-home message from the researchers behind the large cohort study is that even at the highest-risk time points, perforation remains an incredibly rare event, and the benefits of effective contraception generally outweigh the risks.

Expulsion

Expulsion means the IUD partially or fully slips out of the uterus. It is more common than perforation but still affects a minority of users. Younger age is one of the strongest predictors: adolescents aged 14 to 19 had roughly two to three times the expulsion risk of older users, depending on the IUD type.¹⁴ A separate analysis also identified higher body mass index and high parity (having had four or more pregnancies) as risk factors.¹⁵ An expelled IUD no longer provides reliable contraception, so checking your strings periodically (by reaching into the vagina and feeling for the thin threads hanging from the cervix) is a worthwhile habit, especially in the first few months.

Ectopic Pregnancy Risk

Because IUDs are so effective at preventing pregnancy overall, the absolute risk of ectopic pregnancy (a pregnancy that implants outside the uterus, usually in a fallopian tube) with an IUD in place is very low. But when a contraceptive failure does occur with an IUD, a proportionally higher share of those pregnancies end up being ectopic compared to pregnancies without an IUD. A recent large study found that the rate varied by IUD type. The lowest-dose hormonal IUD (13.5 mg) had the highest ectopic rate at about 0.18 per 100 person-years, while the highest-dose hormonal IUD (52 mg) had the lowest at 0.04. The copper IUD fell in between at 0.07.¹⁶ These are all tiny numbers in absolute terms, but if you experience signs of pregnancy while using an IUD (missed period, positive test, abdominal pain), seek care promptly, because ectopic pregnancy is a medical emergency.

IUDs and Infection

An older concern was that IUDs increase the risk of pelvic inflammatory disease (PID). The current understanding is more nuanced. A systematic review found that people who had chlamydia or gonorrhea at the time of IUD insertion did face increased PID risk, but the absolute numbers were still small: 0 to 5% for those with sexually transmitted infections at insertion, and 0 to 2% for those without.¹⁷ The IUD itself does not cause infection. Any elevated risk comes from bacteria being introduced during the insertion procedure, which is why screening for STIs around the time of placement is standard practice. Long-term use does not increase PID risk.

Emergency Contraception

The copper IUD doubles as the most effective form of emergency contraception available. It can be inserted up to five days after unprotected sex. In a prospective study, no pregnancies occurred among 1,963 people who received a copper IUD for this purpose.¹⁸ A more recent study comparing the copper IUD to the 52-mg hormonal IUD for emergency use found that copper IUDs prevented 100% of expected pregnancies, while the hormonal IUD prevented roughly 93% to 96%.¹⁹ The added benefit is that once the copper IUD is in place for emergency purposes, it stays and provides ongoing contraception for up to a decade.

Treating Heavy Periods and Reducing Cancer Risk

Hormonal IUDs have earned a role beyond contraception. For heavy menstrual bleeding, the 52-mg levonorgestrel IUD is often recommended as a first-line treatment before considering surgery. A study of users with documented heavy bleeding found a median reduction in blood loss of over 93% by cycle three and nearly 98% by cycle six.²⁰ For many people with heavy periods, a hormonal IUD can be genuinely life-changing, eliminating the need for hysterectomy or endometrial ablation.

There is also evidence that IUD use is associated with a reduced risk of endometrial cancer. A pooled analysis found that people who had ever used an IUD had about a 19% lower risk of endometrial cancer, and the protective effect grew stronger with longer duration of use and more recent use.²¹ This association has been observed for both copper and hormonal IUDs, though the mechanisms likely differ (local immune response for copper, endometrial suppression for hormonal).

IUDs for Teens and People Who Have Never Been Pregnant

One of the most persistent myths about IUDs is that they are only appropriate for people who have already had children. This has not been true for a long time. A review of the evidence confirmed that IUD use in young, nulliparous people (those who have never given birth) is safe and effective.²² Expulsion rates are somewhat higher in adolescents, and teens are more likely to request early removal because of bleeding or pain. A systematic review of copper IUD continuation in young nulliparous users found that smaller, more flexible IUD designs had notably higher continuation rates, around 86% to 91%, compared to the standard rigid copper T.²³ If you are young and considering a copper IUD, asking about smaller-frame options may make a real difference in your experience.

Fertility After Removal

IUDs do not impair future fertility. One of the earliest and most cited studies on this found that among people who had their IUD removed to get pregnant, over 94% conceived, and more than half of those conceptions happened within the first three months.²⁴ People sometimes wonder whether the hormonal IUD might delay conception more than the copper one because of its suppressive effect on the uterine lining. A study comparing the two found no significant difference: 96% of pregnancies in both groups occurred within the first year after removal, and the cumulative conception rate by 24 months was similar regardless of which device had been used.²⁵ The endometrium recovers quickly once the hormonal IUD comes out.

Removal and Self-Removal

Getting an IUD removed is typically faster and less uncomfortable than insertion. A provider grasps the strings with a small instrument and pulls gently; the arms of the T fold up and the device slides out in seconds. But access to a provider is not always easy, and recent research has explored whether self-removal is safe. A mixed-methods study found that expert clinicians agreed IUD self-removal was low risk, with the key requirements being the ability to feel the strings and a squatting position.²⁶ A randomized trial confirmed self-removal was feasible for motivated users.²⁷ That said, when this was tested in a real-world setting in Brazil, about 73% of participants declined to attempt it, mainly due to fear, and among those who tried, only about 12.5% succeeded, mostly because they could not locate the strings.²⁸ Self-removal is an option if you can feel your strings and are comfortable trying, but it is not for everyone.

Cost-Effectiveness

IUDs have a high upfront cost, which can be a barrier, but over time they are among the cheapest contraceptive methods available. A cost-effectiveness analysis in the United States found that the copper IUD was the least expensive method over its lifespan, followed by vasectomy and the levonorgestrel IUD.²⁹ Analyses from other countries reach similar conclusions. In the UK, a modeling study for national guidelines found that long-acting reversible contraceptives like IUDs dominated combined oral contraceptives, meaning they were both more effective and less costly.³⁰ A Brazilian analysis modeling national health system costs found that the copper IUD was dominant in 100% of simulations, saving over $400 million across a five-year period compared to short-acting methods.³¹ In many insurance systems and public health programs, the upfront cost is covered, making IUDs effectively free. If cost is a concern, it is worth checking whether your insurance or local clinic covers the device and insertion.