The invention relates to oral pharmaceutical compositions for the prevention and/or the treatment of viral diseases. This invention also addresses methods of prevention and/or treatment of these viral diseases, using these oral compositions. One of the main problems considered in the present invention is to enhance the efficiency of anti-viral treatments, especially against Hepatitis C virus by means of ribavirin, for example in combination with interferon. The oral ribavirin antiviral composition according to the invention increases the bio-absorption time of ribavirin, and thus improves the treatment of patients. Said composition comprises at least one modified release form of ribavirin, the bio-absorption time BAT of which is greater than the bio-absorption time BAT* of a reference* immediate release form of ribavirin administered at the same dose; BAT being preferably comprised between 2 and 15 h and more preferably between 4 and 12 h. Said composition is a reservoir type form or a matrix type form. Said composition is a gastric retentive system or a multiparticulate form.