Several common medications can trigger a positive result for tricyclic antidepressants (TCAs) on a drug test, even if you’ve never taken one. The most frequent culprits are the muscle relaxant cyclobenzaprine, the antipsychotic quetiapine, the antihistamine diphenhydramine (Benadryl), and the seizure medication carbamazepine. These false positives happen because standard drug screens use a shortcut method that can’t tell the difference between actual TCAs and other drugs with a similar chemical shape.
Why These False Positives Happen
Most drug screens use a technique called immunoassay, which works by recognizing the general shape of a molecule rather than identifying the exact drug. Tricyclic antidepressants get their name from their three-ringed chemical structure. The problem is that plenty of other medications also have a three-ringed structure, and the test can’t reliably distinguish between them. Research in Clinical Chemistry found substantial overlap between the shape of actual TCAs and other three-ringed molecules like cyclobenzaprine, phenothiazines, and quetiapine, suggesting that cross-reactivity is essentially built into how these tests work, regardless of which specific TCA the test is designed to detect.
Muscle Relaxants
Cyclobenzaprine (sold as Flexeril and Amrix) is the single most common cause of false positive TCA results. Its molecular structure is so close to that of actual tricyclic antidepressants that even older, more advanced lab methods could only distinguish between them by differences of a few seconds in processing time. If you take cyclobenzaprine for back pain or muscle spasms, you should expect it to show up as a TCA on a standard screen. This is well documented and widely recognized in clinical settings.
Antipsychotics
Quetiapine (Seroquel), used to treat schizophrenia, bipolar disorder, and sometimes insomnia, is another well-known source of false positive TCA results. In a study testing three different commercial TCA screening kits, two out of three produced positive results for quetiapine at both therapeutic doses and overdose levels. The third kit did not flag it at all, even at very high concentrations. This means whether quetiapine triggers a false positive depends partly on which brand of test is being used, which can create confusion when results vary between facilities.
Older antipsychotics in the phenothiazine class, such as chlorpromazine, also share enough structural similarity with TCAs to cause cross-reactivity on screening tests.
Antihistamines and Allergy Medications
Diphenhydramine, the active ingredient in Benadryl and many over-the-counter sleep aids, can produce a false positive TCA result. At normal doses this is unlikely to be a problem, but at higher concentrations it becomes a real issue. One documented case involved a patient who ingested a large amount of diphenhydramine and continued to test positive for TCAs for over 60 hours, not converting to a negative result until roughly 90 hours after the initial screen. Cross-reactivity tends to occur when diphenhydramine levels in urine exceed a certain threshold, which is most relevant in overdose situations but can occasionally affect people taking high or frequent doses.
Cyproheptadine (Periactin), an antihistamine sometimes prescribed for appetite stimulation or migraine prevention, has also caused false positives. In at least one pediatric case, cyproheptadine toxicity was initially misdiagnosed as TCA poisoning based on the screen results. Interestingly, the cross-reactivity came not from cyproheptadine itself but from one of its metabolites, the breakdown products the body creates after processing the drug.
Seizure Medications
Carbamazepine (Tegretol), commonly prescribed for epilepsy, nerve pain, and bipolar disorder, is a documented cause of false positive TCA screens. Multiple clinical cases have been reported where carbamazepine ingestion was only identified because the initial toxicology screen came back positive for TCAs, prompting further investigation. If you take carbamazepine and undergo drug screening, this is a predictable result worth mentioning to whoever is ordering the test.
The Full List of Known Offenders
To summarize, drugs that have been reported to cause false positive TCA results on immunoassay screens include:
- Cyclobenzaprine (Flexeril): muscle relaxant, most common cause
- Quetiapine (Seroquel): atypical antipsychotic
- Chlorpromazine and other phenothiazines: older antipsychotics
- Diphenhydramine (Benadryl): antihistamine and sleep aid
- Cyproheptadine (Periactin): antihistamine
- Carbamazepine (Tegretol): anticonvulsant
All of these share enough structural similarity with tricyclic antidepressants to fool the antibody-based detection method used in standard screens.
What To Do About a False Positive
If you test positive for TCAs and you’re not taking one, the standard next step is confirmatory testing. This uses a more precise laboratory method, typically gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-mass spectrometry, that identifies the exact molecules present in your sample rather than just recognizing a general shape. These confirmatory tests are highly selective and can distinguish between an actual TCA and a structurally similar medication without any overlap.
The most practical thing you can do is provide a complete list of every medication you take, including over-the-counter drugs and supplements, before the test is run. Pharmacists and testing professionals recognize that immunoassay screens for TCAs are particularly prone to cross-reactivity. Having your medication list on record gives whoever interprets the results the context they need to flag a likely false positive and order confirmation testing rather than acting on an inaccurate result.