Free-text terms (also called text words) are used by authors in the title and abstract of their studies when published as journal articles; these terms are then searchable in the title and abstract of electronic records in databases. Two important problems severely limit the usefulness of free-text searching:
The wide range of terms authors use to describe adverse effects, both in a general sense (toxicity, side effect, adverse effects) and more specifically (for example, lethargy, tiredness, malaise may be used synonymously);
The free-text search does not detect adverse effects that are not mentioned in the title or abstract of the study and are, therefore, not included in the electronic record (even though the full report describes them) (Derry 2001).
A highly sensitive free-text search should incorporate the potentially wide variety of synonymous terms while also taking into account different conventions in spelling and variations in the endings of terms to include, for example, singular and plural terms. This should then be combined with free-text terms involving the intervention of interest, for example:
(aspirin or acetylsalicylic acid) and (adverse or side or hemorrhage or haemorrhage or bleed or bleeding or blood loss).