Allergies can increase tinnitus, so how would one determine whether it was the allergy or the allergy medication?
Excellent question. You look at data from clinical trials where one group received the drug and the other the placebo and then see if the symptom rates are different between the two groups. (This doesn't pick up post-approval experience in larger populations, but it does provide much better evidence on whether there is a causal or spurious relationship.)
For some drugs this is available in varying levels of detail in the "full prescribing information". Sometimes the drug makers have this on their sites, sometimes it is on the FDA site.
From the prescribing information for Zyrtec, tinnitus was reported less than 2% of the time. For more common adverse reactions they give a comparison to placebo. So for example, 14% of people 215 people on 10 mg of zyrtec reported a headache. Sounds bad until you see that 12.3% of the 309 people on the placebo reported a headache.
Here's the full list of side effects occurring less than 2% of the time in trials of 4000 adults and 650 kids. Sadly we don't know specific percentage or the percentage in placebo groups. None of this is necessarily causal (and most almost certainly isn't - see about about headaches), and pretty much every medication has a list like this. Sometimes it is more detailed and sometimes less.
I've added bold for the hearing categories and for some other potentially 'bad' outcomes.... Here's the list:
Autonomic Nervous System: anorexia, flushing, increased salivation, urinary retention.
Cardiovascular
: cardiac failure, hypertension, palpitation, tachycardia.
Central and Peripheral Nervous Systems: abnormal coordination, ataxia,
confusion,
dysphonia, hyperesthesia, hyperkinesia, hypertonia, hypoesthesia, leg cramps, migraine,
myelitis,
paralysis, paresthesia, ptosis, syncope, tremor, twitching, vertigo, visual field defect.
Gastrointestinal: abnormal hepatic function, aggravated tooth caries, constipation, dyspepsia,
eructation, flatulence, gastritis, hemorrhoids, increased appetite, melena,
rectal hemorrhage,
stomatitis including ulcerative stomatitis, tongue discoloration, tongue edema.
Genitourinary: cystitis, dysuria, hematuria, micturition frequency, polyuria, urinary
incontinence, urinary tract infection.
Hearing and Vestibular:
deafness, earache, ototoxicity, tinnitus.
Metabolic/Nutritional: dehydration,
diabetes mellitus, thirst.
Musculoskeletal: arthralgia, arthritis, arthrosis, muscle weakness, myalgia.
Psychiatric: abnormal thinking, agitation, amnesia, anxiety, decreased libido, depersonalization,
depression, emotional lability, euphoria, impaired concentration, insomnia, nervousness,
paroniria, sleep disorder.
Respiratory System: bronchitis, dyspnea, hyperventilation, increased sputum,
pneumonia,
respiratory disorder, rhinitis, sinusitis, upper respiratory tract infection.
Reproductive: dysmenorrhea, female breast pain, intermenstrual bleeding, leukorrhea,
menorrhagia, vaginitis.
Reticuloendothelial: lymphadenopathy.
Skin: acne, alopecia, angioedema, bullous eruption, dermatitis, dry skin, eczema, erythematous
rash, furunculosis, hyperkeratosis, hypertrichosis, increased sweating, maculopapular rash,
photosensitivity reaction, photosensitivity toxic reaction, pruritus, purpura, rash, seborrhea, skin
disorder, skin nodule, urticaria.
Special Senses: parosmia,
taste loss, taste perversion.
Vision:
blindness, conjunctivitis, eye pain, glaucoma, loss of accommodation,
ocular
hemorrhage, xerophthalmia.
Body as a Whole:
accidental injury, asthenia, back pain, chest pain, enlarged abdomen, face
edema, fever, generalized edema, hot flashes, increased weight, leg edema, malaise, nasal polyp,
pain, pallor, periorbital edema, peripheral edema, rigors.