Long-acting forms of theophylline for use in treating asthma comprise theophylline associated with a compound of the substituted cinnamamide type of the general formula ##STR1## wherein R is hydrogen, a hologen or an acetyl radical; R.sub.1 is hydrogen or the methyl radical; R.sub.2 is ##STR2## wherein R.sub.3 is hydrogen or the methyl radical; or R.sub.1 and R.sub.2 together with the adjacent nitrogen tom form the 2,2-dimethyl-3-oxazolidinyl group.