In the run-in phase of a clinical trial the study subject use the same medicine or not at all; with the treatment phase begins the true clinical trial with the administration of a drug or a placebo.

See e.g.
DRUG FREE PERIOD:
A period during which no study drug is taken. Drug free periods can occur during the run-in phase for a study (when the patients' current medication has been stopped) or it may occur during a wash-out period between two periods of active treatment.
http://www.medistudy.com/clinical_trials/glossary/

Patients assigned to the "active herb" would start to take the placebo/herb for exactly two weeks before therapy (run-in period) to allow the researchers to look for side effects which were due exclusively to the herbal therapy.
http://www.ucsfbreastcarecenter.org/forum/1999/minutjan.html

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Note added at 10 mins (2003-10-04 04:56:26 GMT)
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Run-in period
A period before randomisation that can be used to increase the efficiency of the trial e.g., by examining patients’ tolerance to the study medication or their compliance to taking their treatment.
http://www.europa-trial.org/press/glossary.asp


run-in: all subjects start on a placebo and after some weeks those who have complied with the intervention are randomized blindly to the several study arms; time series design, where each subject serves as her own control during sequential treatment and control periods; crossover design, in which half of the subjects are randomly assigned to the control and the experimental arms, and then are reversed later in the study.
http://www.caps.ucsf.edu/toolbox/Glossary.html


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Note added at 11 mins (2003-10-04 04:57:46 GMT)
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Run-in period
A period before a trial is commenced when no treatment is given. The data from this stage of a trial are only occasionally of value but can serve a valuable role in screening out ineligible or non-compliant participants, in ensuring that participants are in a stable condition, and in taking baseline observations. A run-in period is sometimes called a washout period if treatments that participants were using before entering the trial are discontinued.
http://www.ovid.bids.ac.uk/ovidweb/fldguide/glossary.htm