What is the test for calling a doctor as a witness in a criminal case?

People v. Buckey (1972) 23 Cal.App.3d 740 is instructive. In that case, a defendant was charged with, among other things, unlawful possession of amphetamines. (Id. at p. 741.) The defendant contended that the drug in question had been lawfully prescribed. (Id. at p. 743.) After the trial court sustained objections to defendant's own testimony on this point, his counsel proposed to call his doctor as a witness. (Ibid.) Defense counsel contacted the doctor, who agreed to testify, but he was not available

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