Worsening or precipitation of asthma by beta-adrenoceptor antagonists is well recognized. Severe bronchoconstriction may be induced even in 'mild' asthmatics, and the dose of beta blocker required may be low, as in the case of eye drops of timolol, a nonselective beta blocker used to treat glaucoma. The severity of bronchoconstrictor response is not predictable. Nonselective beta blockers are more likely to precipitate bronchospasms in patients with asthma. The mechanism of beta-blocker-induced asthma is still not certain. Normal subjects develop neither a deterioration in lung function nor an increased bronchial hyperreactivity; therefore, beta blocker drugs should in general be avoided by asthma patients. Safe alternative therapies exist for both hypertension (calcium antagonists, ACE inhibitors, diuretics) and ischemic heart disease (calcium antagonists, nitrates).