Intention-to-treat preserves randomization
ITT keeps every randomized participant in their assigned arm. Because assignment, not adherence, defined the groups, the groups remain exchangeable, and the analysis estimates the effect of being assigned to the treatment strategy.
Per-protocol breaks the randomization
Per-protocol analysis includes only those who adhered. Adherence is often related to prognosis — people who adhere may be healthier or more motivated — so the remaining groups are no longer exchangeable, and the apparent benefit can be an artifact of who adhered rather than an effect of the treatment.
A large per-protocol benefit that shrinks under ITT
Suppose a trial reports a 40% relative risk reduction in the per-protocol analysis but only 10% under ITT. The per-protocol benefit is concentrated in the people who adhered. That could mean the treatment only works in adherent patients, or it could mean that adherence selected a better-prognosis group. To tell these apart, look at why people deviated and whether the deviation was related to the outcome. If the people who stopped the treatment did so because they were deteriorating, the per-protocol group is enriched with people who were doing well, and the 40% is inflated.
The appraisal question
Neither analysis is universally correct. Ask whether the analysis population matches the question the trial claims to answer, and whether the reasons for exclusion are related to the outcome.