A crossover design lets participants experience both treatment conditions, but the order of those conditions can still matter. When a finding depends on sequence, the explanation requires more than calling the second condition effective.
What did participants receive?
Johnson and colleagues’ 2005 laboratory study enrolled 18 adults with methamphetamine dependence who were not seeking treatment. Two eight-day inpatient periods, separated by at least a week, compared oral isradipine with placebo. Isradipine included 30 mg sustained release nightly and 15 mg immediate release two hours before challenge sessions. Intravenous methamphetamine challenges were 0, 15 and 30 mg.
Selected liking and stimulation-related effects were reduced principally when placebo came first and isradipine second. The reverse sequence did not show the same pattern. The investigators proposed conditioning among several explanations, also identifying possible unblinding from adverse effects. Headaches occurred in 15/18 participants during isradipine versus 6/18 during placebo.
What sequence dependence can and cannot establish
The pattern shows that the treatment comparison was not independent of experimental history. It does not identify which aspect of that history produced the difference.
Conditioning is one candidate explanation: an earlier drug experience might change how a later experience is perceived. Expectation, recognition of side effects, period effects or other unmeasured influences could also contribute. Distinguishing these possibilities requires evidence designed to separate them.
It would therefore be too strong to describe this experiment as proving that isradipine works through conditioning. The proposed mechanism is an interpretation of the pattern, not a separately isolated intervention demonstrating that conditioning was necessary or sufficient.
Why the blinding question is substantive
A nominally blinded comparison can still be affected if participants infer their treatment from recognizable effects. A difference in headache experience does not prove that blinding failed, but it supplies a concrete reason to examine that possibility when outcomes depend on subjective judgments.
The correct account preserves the observation and the uncertainty together. Dismissing every rating because unblinding was possible would go beyond the evidence. Ignoring the possibility while claiming an established central mechanism would also go beyond it.
This was a small controlled exposure experiment, not an outpatient trial of sustained methamphetamine abstinence. Its value is in defining a conditional laboratory result and the next questions needed to explain it. It does not establish a generally effective treatment regimen or justify using isradipine to counteract methamphetamine.
Source
Johnson BA, et al. Effects of isradipine, a dihydropyridine-class calcium-channel antagonist, on d-methamphetamine’s subjective and reinforcing effects. Int J Neuropsychopharmacol. 2005;8:203–213. doi:10.1017/S1461145704005036. Publisher full-text PDF.