All enrolled patients have to be a part of the final analysis
This preserves the bias-protective effect of randomisation
Contrasts with per-protocol analysis and as-treated analysis:
Per-protocol analysis: only includes patients who adhered to the protocol; subject to selection bias. May be appropriate for analysis of adverse effects in drug trials.
As-treated analysis: groups patients according to the treatment they received, rather than the treatment they were allocated to receive. Subject to confounding.
Modified intention-to-treat analysis: allows the exclusion of patients which never started treatment for whatever reason. Very subjective, and open to selection bias.
Advantages
A more reliable estimate of treatment effectiveness
Prevents bias
Minimises Type 1 errors (false positives)
Supported by the CONSORT statement
When intention-to-treat analysis agrees with per-protocol analysis, it increases the validity of the study
Disadvantages
Treatment effect is diluted (ends up underestimated)
ITT is inaccurate unless there are negligible protocol violations
ITT alone is inappropriate for non-inferiority trials