Just some miscellaneous comments - it doesn't make any difference what base log transform is used, the estimate for t1/2 should be the same. This is because the various log values differ only by a multiplicative constant, which factors out.
Next, log transforming the data makes some assumptions regarding the variance (that the variance is proportional to the mean, for instance). Consequently, this could be done on untransformed data using PROC NLIN or PROC NLMIXED. By including an indicator variable and some clever programming steps, you could get an estimate of the difference in t1/2 between treatments using an ESTIMATE statement in NLMIXED. If you are interested in this approach, let us know.
Finally, I had no trouble running your code. I assume that the method is historically what has been done in your field, so I would stay with it.
SteveDenham
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