Does duloxetine show up as an illegal drug?

Medications that may cause false positives

1. Dextromethorphan (Robitussin, Delsym)
Dextromethorphan is the main ingredient in many OTC cough suppressants. If you’ve taken dextromethorphan, a urine test may show positive for PCP.

2. Diphenhydramine (Benadryl)
Diphenhydramine is an antihistamine that can help treat allergies, itching, and cold symptoms. Diphenhydramine may cause a false positive result for some opioids and opiates. But it’s not clear what dose of diphenhydramine would cause a false positive result. Typical doses of diphenhydramine may not cause a false positive result.

Some reports have also shown that diphenhydramine may turn a drug test positive for PCP. But this is rare.

3. Certain decongestants
Used for sinus and nasal congestion, pseudoephedrine (Sudafed) may cause false positive test results for amphetamine or methamphetamine.

4. Phentermine
Phentermine (Adipex-P) is FDA-approved to help some people lose weight. It works by lowering your appetite. Taking phentermine can cause false positive urine test results for amphetamines.

5. NSAIDs
Ibuprofen (Motrin, Advil) and naproxen (Aleve, Naprosyn) are in a group of medications called nonsteroidal anti-inflammatory medications (NSAIDs). They’re used to help treat fever, inflammation, and pain. If you take either of these OTC NSAIDs, there’s a small chance your urine test may be positive for barbiturates (a type of sedative) or THC.

Another NSAID, oxaprozin (Daypro), can be used to treat pain from arthritis. Taking oxaprozin may result in a false positive test for benzodiazepines.

6. Some antidepressants
Several antidepressants can cause false positive results on a urine drug test. Examples include:

Venlafaxine (Effexor XR) and desvenlafaxine (Pristiq) are part of a group of antidepressant medications called serotonin-norepinephrine reuptake inhibitors (SNRIs). Taking either of these medications may result in a false positive for PCP.
Sertraline (Zoloft) is a selective serotonin reuptake inhibitor (SSRI). Sertraline can cause a positive result for the hallucinogen LSD on a urine drug test. It may also cause a positive result for benzodiazepines.
Trazodone is an older antidepressant that's not commonly used for depression anymore. Taking trazodone may cause a false positive test result for amphetamine or methamphetamine.
Bupropion (Wellbutrin XL, Wellbutrin SR) may lead to false positive results for amphetamine or methamphetamine.

7. Quetiapine
Atypical antipsychotics can help treat certain mental health conditions like bipolar disorder and schizophrenia. Quetiapine (Seroquel) is one type of atypical antipsychotic. It could cause you to test positive for opioids (specifically methadone).

8. Proton pump inhibitors
Proton pump inhibitors (PPIs) can treat gastroesophageal reflux disease (GERD) and heartburn symptoms. Taking PPIs — specifically pantoprazole (Protonix) — may cause false positive results for THC.

9. Quinolone antibiotics
Quinolone antibiotics treat many bacterial infections. Levofloxacin (Levaquin) and ofloxacin are the most likely of the quinolone antibiotics to cause a false positive urine test for opiates. Ofloxacin may also cause a false positive result for amphetamine or methamphetamine.

10. Promethazine
Promethazine is part of a group of medications called phenothiazines. It can be used to help relieve nausea and vomiting. It can cause false positive results for amphetamine or methamphetamine.

Rare (but possible) medications that can cause false positives
There are a few other medications that may rarely cause false positives. These include labetalol (Trandate), doxylamine (Unisom), and tramadol (Ultram).

Labetalol
Labetalol is in a group of medications called beta-blockers. It helps lower blood pressure in people with high blood pressure (hypertension). While rare, labetalol could cause a false positive test for amphetamine or methamphetamine.

Doxylamine
Doxylamine is an OTC medication that may help people struggling to fall asleep at night. Large doses of doxylamine can cause a false positive for opioids (specifically methadone).

Tramadol
Tramadol is an opioid medication that treats pain. Your drug screen could become positive for methadone or PCP with large doses of tramadol.

Source: GoodRx Health

Just to give you an idea of the medications that can cause false positive test results.

Anything is possible, but duloxetine doesn't show up on a standard drug test and typically doesn't cause false positives unlike several other antidepressants.
There's always the possibility of a lab error and that would need to ruled out.