Adverse reactions may include xerostomia; urinary hesitancy and retention; blurred vision; tachycardia; palpitation;
mydriasis; cycloplegia; increased ocular tension; loss of taste sense; headache; nervousness; drowsiness; weakness;
dizziness; insomnia; nausea; vomiting; impotence; suppression of lactation; constipation; bloated feeling;
musculoskeletal pain; severe allergic reaction or drug idiosyncrasies, including anaphylaxis, urticaria and other dermal
manifestations; and decreased sweating. Acquired hypersensitivity to barbituates consists chiefly in allergic reactions
that occur especially in persons who tend to have asthma, urticaria, angiodema and similar conditions. Hypersensitivity
reactions in this category include localized swelling, particularly of the eyelids, cheeks, or lips, and erythematous
dermatitis. Rarely, exfoliative dermatitis (e.g. Stevens-Johnson syndrome and toxic epidermal necrolysis) may be
caused by phenobarbital and can prove fatal. The skin eruption may be associated with fever, delirium, and marked
degenerative changes in the liver and other parenchymatous organs. In a few cases, megaloblastic anemia has been
associated with the chronic use of phenobarbital. Elderly patients may react with symptoms of excitement, agitation,
drowsiness, and other untoward manifestations to even small doses of the drug. Phenobarbital may produce
excitement in some patients, rather than a sedative effect. In patients habituated to barbiturates, abrupt withdrawal
may produce delirium or convulsions.