— It is unlawful for any person to deliver a legend drug except upon prescription of a physician. RCW 69.41.030. Vernon Clausing is a defrocked osteopathic physician. He delivered a legend drug1 to a police agent. The State charged him with violating former RCW 69.41.030 (1994). The dispositive question is whether the jury instructions correctly specified the elements of former RCW 69.41.030. The trial court effectively instructed the jury that it was a crime to deliver a legend drug unless delivery was by a licensed practitioner. The statute does not require delivery by a practitioner. The instructions were therefore defective. We also conclude that the trial judge should not *623have allowed the executive director of the Washington State Board of Pharmacy to answer the legal question of whether a prescription remains effective after the issuing physician loses his license. We therefore reverse the judgment and sentence.
FACTUAL BACKGROUND
The essential facts here are undisputed.
The Washington State Board of Osteopathic Medicine and Surgery revoked the license of Dr. Vernon Clausing in April 1995 for violating RCW 18.130.1802 by overprescribing the legend drugs carisoprodol (Soma) and nalbuphine (Nubain).
Undeterred, Dr. Clausing hired two licensed physicians to staff his clinic on an occasional basis. He then continued to purchase these drugs, using the prescription drug authorization numbers of the two licensed physicians. Dr. Clausing then continued to supply Soma and Nubain as refills of what he claimed were valid prescriptions issued prior to the revocation of his license to practice.
Sheryl Reynaga is a former patient and former volunteer employee of Dr. Clausing. In July 1995, she reported to the King County police that Dr. Clausing was still distributing legend drugs despite his license suspension. Police enlisted her support for a sting operation. She made three controlled buys from Dr. Clausing while fitted with a recording device. Dr. Clausing delivered Nubain and Soma to her.
Based on this evidence, police executed search warrants on Dr. Clausing’s clinic and home. They seized large quantities of Nubain and several bulk bottles of Soma tablets. The State charged Dr. Clausing with numerous violations of the drug act. He was tried to a jury in 1997. The jury was unable to reach a verdict on three counts of unlawful delivery of a legend drug and one count of possession with *624intent to deliver a legend drug, all contrary to former RCW 69.41.030. The State retried those counts before a jury in 1998. And the jury found Dr. Clausing guilty of all four counts. These guilty verdicts are the subject of this appeal.
TRIAL
Former RCW 69.41.030 provides in pertinent part: “It shall be unlawful for any person to sell, deliver, or possess any legend drug except upon the order or prescription of a physician under chapter 18.71 RCW, an osteopathic physician or an osteopathic physician and surgeon under chapter 18.57 RCW. . . .” The statute contains the proviso that it does not apply to a practitioner acting within his or her license.
Dr. Clausing conceded that he delivered Nubain and Soma, and that these were legend drugs as defined by the statute. He also conceded that he was not licensed as a physician or pharmacist and was not, therefore, a “practitioner” as defined in former RCW 69.41.010(11) (1994). Report of Proceedings (RP) (Feb. 24, 1998) at 39, 44; RP (Feb. 26, 1998) at 85, 86-87.
His defense was that Ms. Reynaga had a valid prescription. RP (Feb. 26,1998) at 83,122. He produced no evidence of a written prescription at trial. Nor could he state who issued the specific prescription here, or when. Id. at 87. Dr. Clausing testified, however, that his medical files had been seized by the State. And, although the State had granted him access to these files, Ms. Reynaga’s most recent medical file had been lost. Id. at 15-16. Ms. Reynaga did not testify. Dr. Clausing insisted that he never delivered legend drugs without a prescription. Id. at 122-23.
Donald Williams is the executive director of the Washington State Board of Pharmacy. He testified, over Dr. Clausing’s objection, that a prescription issued by a physician became invalid when the doctor’s license was revoked. The court instructed the jury that the director’s opinion was not to be taken as a legal opinion, but only from the *625practical perspective of a pharmacist. RP (Feb. 11, 1998 morning) at 15.
The court first instructed the jury that:
It is a crime for any person to deliver a legend drug except upon the order or prescription of a licensed physician or osteopathic physician except as authorized by law.
Clerk’s Papers (CP) at 200 (Instruction No. 7) (emphasis added). This modifies the pattern instruction. The word “physician” is not qualified by “licensed” in the pattern instruction. 11 Washington Pattern Jury Instructions: Criminal 55.03, at 693 (2d ed. 1994) (WPIC). But it accurately states the substance of former RCW 69.41.030.
If the defendant asserts that a prescription exists, WPIC 55.03 directs the court to add the following burden of proof instruction found in WPIC 55.06:
It is not unlawful to deliver a legend drug upon the order or prescription of a physician or osteopathic physician and surgeon.
The State has the burden of proving beyond a reasonable doubt that the defendant did not deliver a legend drug upon the order or prescription of a physician or osteopathic physician and surgeon.
But the court again modified the instruction by importing into WPIC 55.06 language from the professional licensing regulations of chapter 18.64 RCW (Pharmacists) and instructed the jury that:
It is not unlawful to deliver a legend drug if the legend drug either: 1) is dispensed by a licensed practitioner upon the order or prescription of a licensed physician or osteopathic physician; or 2) is delivered by a practitioner acting within the scope of his or her license.
The State has the burden of proving beyond a reasonable doubt that the legend drugs in Counts I, II, and III were not dispensed by a licensed practitioner upon the order or prescription of a licensed physician and that they were not delivered by a practitioner acting within the scope of his or her license.
*626CP at 210 (Instruction No. 17) (emphasis added). A corresponding instruction was given for the possession count. CP at 218 (Instruction No. 25).
Dr. Clausing objected to Instruction No. 17 arguing it was confusing and should be omitted from the instructions entirely: “I just think it confuses everybody. It’s confused you, it’s confused us, it’s confused the jury. We’re not able to argue about what the real facts of the case are if [sic] whether the patient had a prescription.” RP (Mar. 2, 1998) at 24. The court explained that the added language was necessary because the statute was “less than well written.” Id. at 21-22.
The jury found Dr. Clausing guilty as charged.
DISCUSSION
Dr. Clausing contends that the trial court’s burden of proof instruction misstated the requirements of the statute under which he was charged. Specifically, he points out that former RCW 69.41.030 does not make it a crime to deliver without a license to practice. The statute prohibits “any person”—licensed or not—from delivering without a prescription. Dr. Clausing maintains that the challenged instruction substitutes a new element—that a legend drug must be “dispensed by a licensed practitioner” or “delivered by a practitioner acting within the scope of his or her license”—for the actual element of lack of a prescription. CP at 210 (emphasis added). This effectively eliminated Dr. Clausing’s only defense—the existence of a valid prescription—and relieved the State of the burden of establishing the “no prescription” element beyond a reasonable doubt. Standard of Review
Jury instructions are sufficient if they are supported by substantial evidence, allow the parties to argue their theories of the case, and when read as a whole properly inform the jury of the applicable law. State v. Riley, 137 Wn.2d 904, 908 n.1, 909, 976 P.2d 624 (1999). We review the adequacy of jury instructions de novo as a *627question of law. State v. Pirtle, 127 Wn.2d 628, 656, 904 P.2d 245 (1995). It is prejudicial error to submit an issue to the jury that is not warranted by the evidence. State v. Fernandez-Medina, 141 Wn.2d 448, 455, 6 P.3d 1150 (2000).
The Statute
First, it is important to remember that we are not charged with passing on the social or moral propriety of Dr. Clausing’s activities. Conduct may well be harmful, offensive, immoral, or just plain wrong. But whether it is also criminal is a judgment left to the legislature. In re Welfare of Colyer, 99 Wn.2d 114, 139, 660 P.2d 738 (1983). The legal question before us is a narrow one—did the court’s instructions inform the jury of the elements of the crime of unlawful delivery of a legend drug as set out in former RCW 69.41.030 (delivery of a legend drug without a prescription).
Dr. Clausing conceded that he delivered a legend drug. He also conceded that he was not a practitioner. The only factual question before the jury was, therefore, whether Ms. Reynaga had a valid prescription for these drugs. The absence of a physician’s prescription was the single essential element of the offense of unlawful delivery, which the State had to prove beyond a reasonable doubt. Dr. Clausing did not have to prove that Ms. Reynaga did have a prescription. In re Winship, 397 U.S. 358, 90 S. Ct. 1068, 25 L. Ed. 2d 368 (1970) (prosecution must prove the essential elements of the crime and the nonexistence of any defense which by its terms asserts that one of those elements was not present).
The Instructions
But the court’s instructions fail to identify this essential factual dispute for the jury. Instead, the court required the jury to determine whether the legend drugs were dispensed by a licensed practitioner. The net effect was to require both a prescription and a license to practice as conditions for the lawful delivery of legend drugs. Former RCW 69.41.030 does not require this.
*628Instructional error is presumed to be prejudicial unless it affirmatively appears to be harmless. State v. Wanrow, 88 Wn.2d 221, 237, 559 P.2d 548 (1977).
The second prong of the court’s instruction, that a legend drug can be lawfully delivered by “a practitioner acting within the scope of his or her license” is superfluous. CP at 210. Dr. Clausing conceded that he was not a practitioner.
He did testify that he never delivered legend drugs without a prescription. It was up to the jury to accept or reject that assertion. But, given the wording of the instruction, the jury may not have reached that question. Once the jury concluded he was not a practitioner, it could find Dr. Clausing guilty without further inquiry. The result was to relieve the State of the burden of proving an essential element of these crimes. And this requires reversal.
Opinion Testimony on Question of Law
Dr. Clausing next complains that the court improperly allowed the executive director of the Board of Pharmacy to tell the jury that a physician’s prescriptions were no longer valid after the revocation of the physician’s license. This, he says, is a legal opinion. We agree. And, in fact, there are three distinct problems with the testimony.
First, Mr. Williams’ testimony was clearly a legal opinion. A pharmacist’s opinion on the validity of a physician’s prescriptions following a license suspension has relevance only as a question of law.
For an expert to testify to the jury on the law usurps the role of the trial judge. Ball v. Smith, 87 Wn.2d 717, 722-23, 556 P2d 936 (1976). “Each courtroom comes equipped with a ‘legal expert,’ called a judge, and it is his or her province alone to instruct the jury on the relevant legal standards.” Burkhart v. Wash. Metro. Area Transit Auth., 112 F.3d 1207, 1213 (D.C. Cir. 1997). A contrary rule would confuse the jury because “ ‘each party would find an expert who would state the law in the light most favorable to its position.’ ” Harvey Brown, Eight Gates for Expert Witnesses, 36 Hous. L. Rev. 743, 772 (1999) (quoting Askanase v. Fatjo, 130 F.3d 657, 673 (5th Cir. 1997)).
*629Washington Constitution article IV, section 16, provides that the court “shall declare the law.” Legal questions are decided by the court, not the jury, for good reason. By arguing to the court, the lawyers have the opportunity to argue canons of construction; applicable law, including case precedent; and all the other traditional elements that make up legal argument. A judge trained in law then decides whether or not the proposition is legally correct. And he or she can then craft an instruction for the jury. To allow a lay person to answer a legal question puts the lawyers in the impossible position of making these legal arguments to a lay jury.
Second, as the Court of Appeals noted: “The State concedes that this opinion ‘does not appear to be explicitly supported in statutory law,’ and indeed, there is no Washington authority for Williams’ statement.” State v. Clausing, 104 Wn. App. 75, 86, 15 P.3d 203 (2001).
Third, after overruling an objection to the testimony, the court told the jury that:
[T]he question should not elicit and it’s not intended to elicit— and the answers should not be considered by you as a legal opinion—an opinion as to what the law is or what the law is that you would apply in this case.
It’s being offered as an opinion from this witness from the perspective of a pharmacist or an employee of the Board of Pharmacy.
RP (Feb. 11, 1998 morning) at 15. But the testimony is a legal opinion, no matter what the court said, and an erroneous one at that. The evidence was, then, improperly admitted. And the court’s cautionary instruction to the jury was not curative.
The significance of this legal opinion was highlighted by the prosecutor’s jury argument: “The head of the Board of Pharmacy, the man who in this station is responsible for the regulations of these drugs says, When you lose your license to practice medicine, your prescriptions are void.’ ” RP (Mar. 2, 1998) at 101. The court sustained Dr. Clausing’s *630objection to the prosecutor’s remark as a misstatement of the facts. But the opinion nonetheless supported the prosecutor’s argument.
Dr. Clausing’s defense was that he delivered legend drugs only upon a physician’s prescription—including prescriptions issued by him before his license was revoked. The legal validity of these prescriptions was, therefore, critical to his defense. The jury should, therefore, have received instruction on this point, rather than being asked to decide the question as a matter of fact.
CONCLUSION
The conviction is reversed.
Alexander, C.J., Smith, Sanders, and Chambers, JJ., and Kurtz, J. Pro Tern., concur.
Judge Dennis J. Sweeney is serving as a judge pro tempore of the Supreme Court pursuant to Const, art. IV, § 2(a).
A legend drug is one that is required by state law or regulation of the Washington State Board of Pharmacy to be dispensed on prescription only or is restricted to use by practitioners only. Former RCW 69.41.010(9) (1994).
RCW 18.130.180 provides for withdrawal of the license to practice for unprofessional conduct including, among other things, moral turpitude, professional incompetence, and criminal conviction.