Role of acetaminophen (paracetamol, Tylenol) in asthma

Paracetamol use represents a putative risk factor for the development of asthma. There is epidemiological evidence that the risk of asthma may be increased with exposure to paracetamol in the intrauterine environment, infancy, later childhood and adult life.

A dose-dependent association has been observed in different age groups worldwide. An association has also been shown between paracetamol use in both rhinoconjunctivitis and eczema.

There is biological plausibility with paracetamol use leading to decreased glutathione levels resulting in increased oxidant-induced inflammation and potentially enhanced T-helper type 2 responses.

Patterns of paracetamol use might explain the worldwide variation in the prevalence of asthma, particularly the high rates in English-speaking countries, which have high per capita prescription and over-the-counter use of paracetamol.

An association may exists between the international trends of increasing paracetamol use and increasing prevalence of asthma over recent decades.

Randomized-controlled trials (RCTs) are required to determine if there is a causative association.

It is too early to draw any conclusions at this point and I would not recommend a change in the use of acetaminophen for upper respiratory tract infections with fever, pain, or for other indications.

Febrile respiratory illnesses in infancy and atopy are risk factors for asthma http://goo.gl/G3W7J - Children with fever also get Tylenol...

References:

The role of paracetamol in the pathogenesis of asthma. Clin Exp Allergy. 2010 Jan;40(1):32-41. Authors: Farquhar H, Stewart A, Mitchell E, Crane J, Eyers S, Weatherall M, Beasley R.
http://www.ncbi.nlm.nih.gov/pubmed/20205695?dopt=Abstract
Paracetamol use does not increase the risk of asthma - BMJ http://goo.gl/wtKZ
Acetaminophen no asthma trigger after all. http://goo.gl/kHDu

Acetaminophen use may represent an important risk factor for the development and/or maintenance of asthma, rhinoconjunctivitis, and eczema in adolescent children. American Journal of Respiratory and Critical Care Medicine Vol 183. pp. 171-178, (2011).
Paracetamol (acetaminophen) and Antibiotics in Childhood and Later Wheezing/Asthma: Association or Causation? Medscape, 2011.
"Acetaminophen phobia" is not warranted until we see confirmatory studies in children with asthma. Journal Watch, 2012.