1. The Research Question
Is excessive exposure to menthol via cough drops associated with increased cough severity?
Based on observations of a primary care MD:
Some adults with unexplained persistent cough consumed large amounts of cough drops, and the coughs disappeared after stopping the drops
This is an example of TRANSLATING PRACTICE INTO RESEARCH
Johnson D, Mead R, Kennelty K, Hahn D. Menthol Cough Drops: Cause for Concern? JABFM 2018:31:

2. What the Researchers Did
Literature search on menthol
Single dose studies confirm menthol lowers cough
No multiple/high dose studies (e.g., could there be a rebound effect similar to Afrin® nasal spray?)
Menthol toxicity syndrome
Survey of patients with acute/subacute cough
548 adolescent & adult patients in five clinics
Cough duration, severity (7-point scale), type, and amount of cough drops
Clinics were members of the Wisconsin Research & Education Network (WREN) and represented urban, suburban and rural areas

3. What the Researchers Found
Majority (2/3) took cough drops; 90% of cough drops contained menthol
Cough drop use (v non-use) was associated with severity, as expected
Menthol amount was associated with severity
dose per cough drop (R=0.19, P=0.007)
# menthol drops per day (R=0.20, P=0.002)
total daily menthol dose (R=0.21, P=0.001)
Cough drop use (v non-use) was also associated with cough duration AT PRESENTATION. Due to the cross-sectional nature of the project we do not know the actual duration of the entire cough episode, so the duration we measured is more a measure of health seeking behavior than of the natural history of the cough itself.

4. Cough Severity vs. Daily Menthol Dose
Note that some individuals reported taking up to 100 millligrams (mg) of menthol daily. Given that most drops contain 2, 4, or 6 mg this indicates a bag or more of drops daily. An animal model demonstrated lung pathology after a single 200 mg dose of menthol (Abanses et al. Chest 2009;135: )
(R=0.21, P=0.001)

5. What This Means for Clinical Practice
Clinical PEARL
Ask specifically about cough drop use when interviewing patients with an unexplained cough (some do not consider cough drops an OTC medication); consider recommending against excessive use
Future Research
Healthy volunteer studies on the effect of multiple/chronic dose menthol exposure and cough reflex
PEARL: The clinician who initiated this project observed that many patients do not consider cough drops an OTC medication because they do not swallow them.
Future Research: Several experimental models exist to study anti-tussive effects, e.g. nebulized citrate-induced cough model. What has not been reported are effects of, e.g., multiple dose exposure to inhaled menthol over hours/days or weeks.