False-positive drug test? How to check your medicines
An unexpected positive on a home drug test can leave you looking through the medicine cupboard for an explanation. A false-positive drug test result is possible. But a test can also pick up a medicine you've taken exactly as prescribed.
Start with the name beside the strip that gave the result. Then get the test leaflet and medicine packet together. They can help you work out what the screen might have detected, what to write down and what to ask about next.
At a glance
- A drug screen can pick up a prescribed medicine. That alone does not show misuse.
- Some substances can cross-react with a screen and cause a false positive.
- Check any medicine warning against your test's drug panel and sample type.
- Keep the kit details and ingredient labels for medicines and supplements together.
- Take prescribed medicine as directed unless your prescriber advises a change.
- Repeating a home test gives you another screen, rather than laboratory confirmation.
What can an unexpected drug test result mean?
A home drug test gives an initial screening result. Each panel looks for a drug, a group of drugs such as opiates, or a metabolite, which is a substance the body makes as it processes a drug. The panel also has a cut-off: the concentration threshold it uses to report a negative or preliminary positive result.
There are two different reasons a medicine might be involved:
- Genuine detection: the screen picks up an ingredient in your medicine, or a metabolite it is designed to detect.
- Cross-reaction: another substance reacts with the screen and makes it look as though the target substance is present. This can produce a false positive.
Take codeine. A painkiller containing it can register on an opiates screen because codeine is itself an opiate. Lab Tests Online UK explains how medicines can register on a drug screen. That result alone cannot tell you someone has misused their medicine.
Sertraline, an antidepressant, is a different example. Its UK prescribing information reports false-positive urine screens for benzodiazepines with immunoassays, tests that use antibodies to detect substances. The warning is worth checking, but it does not mean every home kit is affected or that sertraline explains your result.
You can read more in our FAQ on medicines and everyday substances that can affect a home drug test.
Your medicine and drug test record
A note on your phone or a page in a notebook will do. Keep the details below in one place, and write down each panel's result separately using the terms in the kit's result diagram.
| Record | Details to keep |
|---|---|
| Test and sample | Manufacturer, exact kit, urine or saliva, batch and expiry |
| Result | Affected drug panel, result and control indicator; collection date and time, and reading time |
| Medicines | Active ingredients, strength, dose and time last taken, including occasional treatments |
| Supplements | Product name and full ingredient label |
| Any problems | Anything that went wrong with sampling, storage or timing |
These are private health details. If the test is for work or another formal process, ask the testing provider how to share your medicine information confidentially.
How to check your medicines against a drug test
Work through the result diagram before looking for a medicine that could explain the result. A reading or timing mistake needs a different response from a possible cross-reaction.
1. Check the control indicator and reading time
Put the affected panel beside the leaflet's result diagram. Our guide to what the lines on a multi-drug test mean can help you make sense of the patterns. With a multi-drug test kit, read each panel separately. A positive on one does not describe the other panels.
If the result is invalid, follow the kit's instructions. Ignore changes after the final reading deadline. And if you're unsure about how you collected the sample or timed the test, go back through our home drug test checklist with the leaflet beside you.
2. Find the active ingredients on the medicine packet
Start with the active ingredients, rather than the brand name on the front. Combination medicines contain more than one. Co-codamol, for example, contains paracetamol and codeine, so write down both, along with the strength, dose and time you last took it.
Include the occasional painkiller or cold remedy as well as regular prescriptions. Keep taking prescribed medicine as directed unless your prescriber advises a change.
3. Check the evidence for your medicine
A list of medicines that cause a false positive drug test may look like an answer. Before relying on it, check which drug group, sample type and test method the claim concerns. A finding from a urine test may not apply to a saliva kit or a different brand.
In the kit leaflet, look for “cross-reactivity”, “interference” or “limitations”. If it says a substance caused “no interference”, check the concentration it was tested at, too. Keep those details with the finding; they tell you how far it applies.
Can't find the information? Give the manufacturer or supplier the exact kit details and ask for its interference information. A pharmacist or clinician can check the medicine's prescribing information too.
4. Can supplements affect drug test results?
Include supplements in your notes, with the product name and full ingredient label. Photographing the label is easier than copying a long list. As with medicines, check any claimed effect against the test you used. Having taken a supplement does not, on its own, explain a positive.
Does a second home drug test confirm the result?
You might feel like taking another test straight away. It will give you another screening result, but even two matching results do not provide laboratory confirmation.
A confirmatory laboratory test uses a more specific method to check for particular substances. Ask what analysis the laboratory will carry out. Laboratories can run initial screens too, so “sent to a lab” does not tell you the whole process.
A later negative does not prove the first result was a false positive either. You may have collected the sample at a different time, or used a test with a different panel or cut-off. Our FAQ explains why home and laboratory drug test results can differ.
If you dispute the result, or it could affect your treatment, employment or a legal decision, ask how it will be confirmed and how the sample needs to be collected.
Who to ask about an unexpected result
For a home test you've taken yourself, a pharmacist or clinician can help you decide what follow-up makes sense. For a workplace test or another formal process, contact the organisation that arranged it.
Take your kit details and medicine record with you, or have them beside you when you call. These questions can help:
- Could this medicine genuinely register on this drug panel?
- Is there evidence of a cross-reaction with this test?
- Is this a preliminary screen or a confirmed finding?
- What confirmation process applies, and what sample does it need?
Before collecting or sending another sample, agree the follow-up with the service providing it. Use its collection instructions, even if you've taken a home test before.
