[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Epidemiology](https://durgical.com/en/epidemiology/) › [Study designs](https://durgical.com/en/epidemiology/areas/designs/) › Crossover trial

# Crossover trial

- Study design

Each patient gets both treatments, one after the other in random order: they are their own control. It suits chronic, stable conditions.

Updated Nov 5, 2026

## Key points

- Each patient is their own control: fewer participants are needed.
- The order of treatments is randomised (AB or BA).
- A washout period between treatments prevents carry-over.
- Only for chronic, stable conditions with fast, reversible effects.
- Not for irreversible outcomes (death, cure) or diseases that change quickly.

## What it is

In a crossover trial each participant receives both interventions in successive periods, with the order assigned at random and a washout period in between. Because each person is compared with themselves, variability between patients is removed and fewer participants are needed than in a parallel trial. It only works in chronic, stable conditions with reversible effects; the main danger is carry-over of the first treatment into the second period.

## How it is built

- The order is randomised: half receive A then B; the other half B then A.
- First treatment period, with outcome measurement.
- Washout period without treatment.
- Second period with the other treatment and a new measurement.
- Differences within each patient are analysed (paired analysis).

## Worked example

- 24 adults with perennial allergic rhinitis: 12 get an active nasal spray (A) then placebo (B); the other 12 get B then A.
- Each period lasts 4 weeks, with a 2-week washout: total time per patient = 4 + 2 + 4 = 10 weeks.
- The mean symptom score (0 to 10 points) is 4.0 on A and 6.5 on B.
- Mean within-patient difference: 4.0 − 6.5 = −2.5 points in favour of A.
- The effect is checked to be similar in both orders (AB and BA) to rule out carry-over.
- Reading: with only 24 patients, each compared with themselves, A lowered symptoms by 2.5 points.

## Crossover versus parallel trial

| Feature | Crossover | Parallel |
| --- | --- | --- |
| Comparison | Within each patient | Between different groups |
| Sample size | Smaller | Larger |
| Suitable conditions | Chronic and stable | Any |
| Own risks | Carry-over and period effects | Imbalance between groups in small samples |
| Time per patient | Longer (two periods and washout) | Shorter |

## When to use it

- Used in stable asthma, chronic pain, hypertension, rhinitis and bioequivalence studies in healthy volunteers.
- Drop-outs weigh more: a patient who leaves during the second period loses their comparison.

## Common pitfalls

- Using it for diseases that resolve or progress: the second period no longer compares like with like.
- A washout too short for a long-acting treatment: carry-over contaminates the second period.
- Analysing the data as two independent groups instead of a paired analysis.

## Memory aid

Crossover is ‘me versus me’: one treatment, a pause to wash out, then the other.

## Test yourself

Think of the answer, then tap the question to see it.

## What is the main advantage of a crossover trial?

Each patient is their own control, which reduces variability and allows fewer participants.

## What is carry-over?

The persistence of the first treatment's effect into the second period, which biases the comparison.

## Why is a crossover trial unsuitable for an acute infection that resolves?

Because after the first period the patient no longer has the disease, so there is nothing to compare in the second.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/epidemiology/crossover-trial/)

## Connections

## Related 4

- [Randomised controlled trial](https://durgical.com/en/epidemiology/randomised-controlled-trial/)
- [Randomisation and allocation concealment](https://durgical.com/en/epidemiology/randomisation-and-allocation-concealment/)
- [Blinding](https://durgical.com/en/epidemiology/blinding/)
- [Superiority, equivalence and non-inferiority trials](https://durgical.com/en/epidemiology/superiority-equivalence-and-non-inferiority-trials/)

[← All of epidemiology](https://durgical.com/en/epidemiology/)

Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.

Sources: [Chapter 23: Including variants on randomized trials](https://www.cochrane.org/authors/handbooks-and-manuals/handbook/current/chapter-23) (Cochrane, 2024); [Chapter 8: Assessing risk of bias in a randomized trial](https://www.cochrane.org/authors/handbooks-and-manuals/handbook/current/chapter-08) (Cochrane, 2024).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/epidemiology/crossover-trial/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/crossover-trial/