An infection linked to an IUD typically produces a strong, fishy smell that is noticeably different from normal vaginal odor. That fishy quality comes from specific chemicals produced by overgrown bacteria, and it is most closely associated with bacterial vaginosis, which IUD users develop at higher rates than users of hormonal contraceptives. A fishy smell on its own does not necessarily mean something dangerous is happening, but it is a reliable signal that the vaginal bacterial balance has shifted and deserves attention. More serious infections, like pelvic inflammatory disease, sometimes involve a different kind of smell or no unusual odor at all, which makes the picture more complicated than “bad smell equals infection.”
Why IUDs Change Vaginal Odor
The vagina maintains its own ecosystem of bacteria, dominated in most people by Lactobacillus species that keep the environment acidic and inhospitable to harmful organisms. An IUD is a foreign object sitting inside the uterus with a thin string trailing through the cervix into the vaginal canal. That string, and the device itself, can shift the microbial balance in a few ways. One longitudinal study found that IUD users had a bacterial vaginosis rate of 37% compared to about 19% among people using hormonal methods like the pill, ring, or patch.1PubMed Central. Risk of Bacterial Vaginosis in Users of the Intrauterine Device: A Longitudinal Study That gap narrowed after adjusting for other factors, but the trend is real and consistent with what many IUD users report experiencing.
Copper IUDs in particular have drawn attention from researchers. There are two leading theories for why they shift the microbiome. First, the chronic presence of a foreign body may create surfaces that BV-associated bacteria like Gardnerella vaginalis colonize more easily. Second, copper IUDs tend to make periods heavier and longer, especially in the first several months. The extra menstrual blood provides iron-rich nutrients that fuel the growth of bacteria associated with dysbiosis.2Frontiers in Microbiomes. The impact of contraceptives on the vaginal microbiome in the non-pregnant state The evidence is mixed on whether hormonal IUDs carry the same risk, though they are generally considered to be somewhat more microbiome-friendly because they thin the uterine lining and reduce menstrual flow over time.
The Chemistry Behind the Fishy Smell
That distinctive fishy odor is not just a vague description. It traces to a specific group of chemicals called biogenic amines, particularly putrescine, cadaverine, and tyramine. These are produced by anaerobic bacteria that thrive when lactobacilli lose their foothold. In a study of 37 women, all three amines were found at significantly higher levels in women whose vaginal microbiome was dominated by a diverse mix of non-Lactobacillus species compared to women with Lactobacillus-dominant communities.3PubMed Central. Vaginal biogenic amines: biomarkers of bacterial vaginosis or precursors to vaginal dysbiosis? The names alone give you a hint about the smell: cadaverine is named after cadavers, and putrescine after putrefaction. At the concentrations found in BV, the result is that unmistakable fishy quality.
The smell often becomes more noticeable after sex or during your period. That is because both semen and menstrual blood are alkaline, and the biogenic amines volatilize more easily in a less acidic environment, meaning they become airborne and hit your nose more readily. If you notice a fishy smell that intensifies specifically after intercourse, that pattern is a classic hallmark of BV rather than something more serious.
What Different Infections Actually Smell Like
Not every IUD-related infection produces the same odor, and some produce very little. BV is by far the most common cause of a noticeable fishy smell in IUD users, but it is technically a disruption of the vaginal microbiome rather than a true infection. It is not caused by a single invading pathogen. It is an overgrowth of bacteria that were already present in small numbers.
Pelvic inflammatory disease, which is the infection people most worry about with IUDs, is a different story. PID involves bacteria ascending from the cervix into the uterus, fallopian tubes, or surrounding tissues. It can be caused by sexually transmitted organisms like gonorrhea and chlamydia, or by a mix of bacteria already living in the reproductive tract. When PID involves vaginal discharge, the discharge is often described as yellow-green or gray, and may have a foul or unpleasant smell, but it is not typically the same clean “fishy” odor associated with BV. Some people with PID have no unusual smell at all, which is part of what makes it tricky to catch early.
Yeast infections, which are not specifically linked to IUD use but can happen to anyone, produce a thick, white, cottage-cheese-like discharge that tends to smell yeasty or bread-like, not fishy. If what you are smelling is more like bread dough than fish, that points away from a bacterial problem and toward yeast overgrowth.
The IUD String as a Bacterial Highway
One of the more interesting findings in recent research involves the IUD string itself. The thin thread that hangs through the cervix serves as a scaffold that bacteria can colonize, forming what is essentially a biofilm, a structured community of microorganisms attached to a surface. A study comparing IUD strings from women who had their devices removed due to symptoms versus women who had routine removals found that the symptomatic group had roughly 25 times more total bacteria on their strings. Critically, the Lactobacillus levels were about the same in both groups, but the non-Lactobacillus species were dramatically elevated in the symptomatic group, including potentially harmful genera like Fusobacterium and Haemophilus.4PubMed Central. Microbial load and composition of intrauterine device string correlates with symptomatic removals in parous women
This suggests that when problems develop, it is not because the protective bacteria disappear. Instead, unwelcome species pile on top of them. The string provides a continuous physical pathway between the vagina and the uterus, which may be part of why some researchers have studied whether string design or material could reduce bacterial colonization. For you as a user, the practical takeaway is that the string is worth paying attention to. If you are checking your strings regularly, as most providers recommend, and you notice a change in smell at the same time, that combination of observations is worth reporting.
How Common Are Serious IUD Infections, Really?
The fear of a serious uterine infection is one of the biggest concerns people have before getting an IUD, and the reality is much less alarming than the worry. In a prospective study of nearly 1,900 women who received a hormonal IUD, only nine developed a pelvic infection over two years of follow-up, a rate of about 0.5%. Those nine infections were spread across the entire two-year period, with no clustering around the time of insertion.5PubMed. A prospective assessment of pelvic infection risk following same-day sexually transmitted infection testing and levonorgestrel intrauterine system placement That finding contradicts the older belief that the insertion procedure itself is the primary risk window for infection.
The bacteria that show up when IUD-related pelvic infections do occur are a diverse group. Studies culturing bacteria from IUDs removed during PID episodes have identified organisms like E. coli, Enterococcus, coagulase-negative Staphylococcus, and in some cases Neisseria gonorrhoeae.6PubMed. Bacterial cultures from intrauterine devices removed from patients with pelvic inflammatory disease7PubMed. Bacteriological cultures of removed intrauterine devices and pelvic inflammatory disease These are common organisms that can cause trouble in the right circumstances, not exotic pathogens. Whether they produce a distinct odor depends on the specific bacteria involved and how far the infection has progressed.
When Smell Is Not the Main Warning Sign
If you are relying on smell alone to tell you whether something is wrong with your IUD, you could miss a serious problem. Bacterial vaginosis, the main source of fishy odor, is uncomfortable and worth treating, but it is not dangerous in the way PID can be. PID, on the other hand, can damage your fallopian tubes and affect future fertility if left untreated. The warning signs for PID are more about pain and systemic symptoms than smell.
Symptoms that should prompt a call to your provider include:
- Pelvic pain: dull or sharp pain in your lower abdomen, especially if it is new or different from normal cramping
- Fever: a temperature above 100.4°F (38°C) alongside any pelvic symptoms
- Painful sex: deep pain during intercourse that was not there before
- Abnormal discharge: yellow, green, or gray discharge that looks different from your usual pattern, with or without a smell
- Irregular bleeding: spotting or bleeding between periods, especially if accompanied by pain
A fishy smell without any of these accompanying symptoms is more likely BV than PID. But the two can overlap, and BV itself may increase vulnerability to ascending infections, so treating it rather than ignoring it is still a good idea.
Does the IUD Need to Come Out?
One of the first questions people ask when they suspect an IUD-related infection is whether the device needs to be removed. The answer depends on the severity of the infection. For BV, removal is almost never necessary. BV is treated with antibiotics, and the IUD can stay in place.
For PID, the picture is more nuanced than you might expect. A systematic review of studies comparing IUD removal to IUD retention during PID treatment found mixed results. One trial showed that women who kept their IUD actually had shorter hospital stays, while another found that women whose IUD was removed had better recovery of clinical signs and symptoms. Across the studies, there were no clear differences in PID recurrence rates or subsequent pregnancy rates between the two groups.8Contraception. Retention of intrauterine devices in women who acquire pelvic inflammatory disease: a systematic review Current guidelines generally allow the IUD to stay in while antibiotics are given, with reassessment after 48 to 72 hours. If there is no improvement, removal becomes more likely.
The fact that removal is not automatic is reassuring for people who rely on their IUD for contraception and do not want to lose that protection during treatment. But the decision is always case-by-case and depends on how you are responding to antibiotics.
Actinomycosis and Long-Term IUD Use
There is a rare but distinctive infection associated specifically with keeping an IUD in place for many years beyond its recommended duration. Actinomyces bacteria, which are slow-growing anaerobic organisms, can colonize the surface of an IUD over time. About 7% of IUD users may have Actinomyces-like organisms show up on a Pap test, but this is usually an incidental finding with no clinical significance, and in the absence of symptoms, it does not require treatment or IUD removal.9PubMed. IUDs and colonization or infection with Actinomyces
Actual pelvic actinomycosis, however, is a different matter. It is extremely rare but has been linked to IUDs left in place for a decade or more. Case reports describe patients who had retained or degraded IUDs in place for over ten years and developed abscesses or masses that mimicked ovarian cancer on imaging.10PubMed Central. Three cases of ovarian actinomycosis with literature review11PubMed Central. Pelvic Actinomycosis Related to Intrauterine Device: A Case Report The chronic irritation of the uterine lining by the device is thought to break down tissue barriers and allow the bacteria to invade deeper. Actinomycosis produces a thick, often yellowish discharge that may have a foul smell, but the odor is not the hallmark symptom. The bigger concerns are pain, weight loss, and a pelvic mass.
Replacing your IUD on schedule, typically every three to ten years depending on the type, effectively prevents this scenario. The condition is almost exclusively seen in people who far exceed the recommended duration of use.
Smell Without an IUD Problem
Before concluding that your IUD is causing a smell, it is worth considering that vaginal odor fluctuates naturally. Menstrual blood has a metallic, iron-rich smell. Sweat and normal discharge have their own mild scent that changes throughout the cycle. A slight change in smell after getting an IUD can simply reflect the heavier bleeding many people experience in the first few months, especially with copper devices. The blood itself creates a temporary environment that favors odor-producing bacteria without necessarily leading to BV.
Retained foreign bodies other than the IUD itself, such as a forgotten tampon, can also produce a dramatic foul smell that gets wrongly attributed to the IUD. If the odor comes on suddenly and is overwhelmingly strong, that possibility is worth checking before assuming an infection. Providers regularly see patients who discover the cause was something mechanical rather than microbiological.
Diet, hydration, and recent antibiotic use can all influence vaginal odor as well. Antibiotics in particular can wipe out protective lactobacilli and trigger BV as a secondary effect, which might have nothing to do with your IUD even though the timing coincides with having one. The key is the character and persistence of the smell: a fishy odor that lasts more than a few days, especially if accompanied by thin grayish discharge, is the pattern that suggests BV and warrants treatment. A transient change in smell during your period or after a course of antibiotics is more likely a temporary fluctuation.
What Irregular Bleeding Signals
Smell and bleeding patterns are connected in IUD users. The longitudinal study that tracked BV rates found that women who reported any irregular bleeding during the study period were more than twice as likely to develop BV, even after adjusting for other risk factors.1PubMed Central. Risk of Bacterial Vaginosis in Users of the Intrauterine Device: A Longitudinal Study This makes biological sense: blood raises vaginal pH and provides nutrients for the anaerobic bacteria that produce odor-causing amines. If you are experiencing both irregular bleeding and a fishy smell, those two symptoms are likely being driven by the same underlying shift in your vaginal environment.
This also means the odor issue often improves on its own as your body adjusts to the IUD. Copper IUD users who experience heavy, prolonged periods in the first three to six months frequently see their bleeding stabilize over time, and the microbiome tends to follow. If you are in the early months after insertion and dealing with both heavier bleeding and a new odor, it may be worth waiting it out while staying alert for any of the more concerning PID symptoms described above. Your provider can prescribe BV treatment in the meantime if the smell is bothersome or if testing confirms the diagnosis.