William M. Bell was charged with two counts of unlawfully selling dangerous drugs. The trial resulted in a jury verdict of guilty as to both counts. The defendant appeals.
The defendant is an osteopathic physician with offices in Seattle. Two police officers came to his office and asked for some amphetamine tablets by a trade name. When the doctor asked if they were patients, they replied they were, and the doctor then sold each 30 tablets of the drug they had requested.
The chronology of events is essential to a discussion of the issues raised:
1. March 1,1971, the sale of the drugs.
2. March 9, 1971, the filing of the charge in district justice court 'and the arrest of the defendant.
3. May 21, 1971, RCW 69.40.060 was repealed effective this date. This statute was in effect at the time of the sale of the drugs. It read in pertinent part:
It shall be unlawful for a person, ... to sell, . . . any of the foregoing substances, ... or any other drug which is required by any applicable federal or state law or federal regulation . . . to be used only on prescription, .except upon the written or oral order or prescription of a physician, '. . . and shall not be refilled without the written or oral order of the prescri-ber: Provided, That the provisions of this section shall not apply to the sale at wholesale by drug jobbers, . . . or to physicians . . ’ . licensed to practice in this state.
4. May 21, 1971, RCW 69.50.308 was not in effect until this date. It reads in part:
(a) Except when dispensed directly by a practitioner authorized to prescribe or administer a controlled substance to an ultimate user, no controlled substance in *672Schedule II may be dispensed without the written prescription of a practitioner.
(d) A valid prescription or lawful order of a practitioner, in order to be effective in legalizing the possession of controlled substances, must be issued in good faith for a legitimate medical purpose by one authorized to prescribe the use of such controlled substance. An order purporting to be a prescription not in the course of professional treatment is not a valid prescription or lawful order of a practitioner within the meaning and intent of this chapter; and the person who knows or should know that he is filling such an order, as well as the person issuing it, can be charged with a violation of this chapter.
(e) A controlled substance included in Schedule V shall not be distributed or dispensed other than for a medical purpose.
5. April 15, 1971, arraignment and entry of plea by the defendant in district justice court.
6. May 18, 1971, the filing of the information in superior court.
7. July 27,1971, trial of the case.
In effect at the time of the sale of the drugs on March 1, 1971, was also RCW 69.40.064. It reads as follows:
A prescription, in order to be effective in legalizing the possession of dangerous drugs, must be issued for a legitimate medical purpose by one authorized to prescribe the use of such dangerous drugs. An order purporting to be a prescription issued to an addict or habitual user of dangerous drugs, not in the course of professional treatment is not a prescription within the meaning and intent of this section; and the person who knows or should know that he is filling such an order, as well as the person issuing it, may be charged with violation of this chapter. A legitimate medical purpose shall include use in the course of a bona fide research program in conjunction with the hospital or university.
To reiterate, on March 1, 1971, the date of the alleged crime, RCW 69.40.060 and 69.40.064 were in effect and applicable, and RCW 69.50.308 was not.
*673The defendant objects to the instruction given which reads:
Under the laws of the State of Washington, sale or dispensing of a dangerous drug is a crime, except upon the written or oral order or prescription of a physician, surgeon, dentist, or veterinary surgeon duly licensed to practice in the State of Washington.
However, a physicain [sic] may sell or dispense dangerous drugs if in good faith and in the course of his professional practice for theraputic [sic] purposes only.
The instruction given informed the jury that a physician could sell a dangerous drug on oral order if he did so:
1. in good faith,
2. in the course of his professional practice, and
3. for therapeutic purposes only.
The requirements enumerated in the instruction for sale by a physician to be legal were not the requirements that had to be fulfilled in order for a physician’s sale to be legal on March 1, 1971, the date of the commission of the alleged crime. On that date, the provisions of RCW 69.40.064 only required that a legitimate medical purpose exist in order for a physician to sell on oral order. In this case, the evidence discloses that the sale was from the physician, as seller, to the officers who claimed to be patients, as purchasers. The instruction improperly informed the jury that the physician’s authority to sell drugs was limited to an extent greater than the statute applicable on March 1, 1971, actually limited such authority. The evidence could indicate negligent and careless conduct on the part of the physician when the drugs were sold, rather than criminal intent, if considered in the light of whether his conduct was for a “legitimate medical purpose.” In the context of this evidence to instruct that the doctor was permitted to sell only for therapeutic purposes required the jury to find that unless the actions of the doctor showed a therapeutic purpose, his act was criminal. This was not the law at the time of the commission of the act.
A therapeutic purpose is defined as a purpose relating to the treatment of disease by remedial agents or meth*674ods. It implies that a curative or healing effect will follow. Dorland’s Illustrated Medical Dictionary (23d ed. 1958). It carries a quasi-medical connotation. See RCW 18.57.170 (9). There is no such connotation to the term “legitimate medical purpose.” “Legitimate” means lawful or legal. Webster’s Third New International Dictionary (1969). In the context of the statute the term “legitimate” does not impose specific restrictions upon conduct by a physician in selling drugs. The term is not a narrowing term as is the term “therapeutic.” We envision that to a layman a medical purpose could include research, diagnosis, tranquilization, stabilization, relief of pain and experimentation to find relief for a patient. These aspects are partially or totally excluded when a “therapeutic” purpose is required.
Criminal statutes are to be strictly interpreted, cannot be extended by intendment and doubts as to whether conduct was criminal must be resolved in favor of a defendant. 1 R. Anderson, Wharton’s Criminal Law & Procedure § 19, at 40 (1957). RCW 69.40.060 and 69.40.064 cannot be construed as saying the same thing as RCW 69.50.308 in different form. While these statutes dealt with the same subject matter and should be construed harmoniously if possible, when a change is enacted and one statute is repealed and another takes its place the new supersedes the former law. The doctrine of pari materia does not require the construction of that which the legislature has laid to rest as having the same meaning as the statute that has taken its place. See 2 J. Sutherland, Statutory Construction §§ 5201, 5202 (F. Horack, Jr. 3d ed. 1943). Where a material change is made in the wording of a statute, a change in legislative purpose must be presumed. Graffell v. Honeysuckle, 30 Wn.2d 390, 191 P.2d 858 (1948).
A newly enacted statute operates prospectively only unless a retroactive effect was the clear intent of the enacting authority. State v. Ladiges, 63 Wn.2d 230, 386 P.2d 416 (1963). However, an act which enlarges the prohibited area of conduct cannot be applied to make criminal that which was previously innocent. Such a construction of an *675act would violate article 1, section 9 of the United States Constitution and article 1, section 23 of the Washington State Constitution prohibiting ex post facto laws. Andrus v. McCauley, 21 F. Supp. 70 (E.D. Wash. 1936); State v. Han-len, 190 Wash. 563, 69 P.2d 806 (1937); State v. Lopeman, 143 Wash. 99, 254 P. 454 (1927).
With the principles set forth in mind, we hold it was reversible error to instruct in the terminology of RCW 69.50.308 which was not in effect on the date of the crime. The instruction subjected the defendant to the risk (a) that conduct not then forbidden could be found to have been forbidden; and (b) that the defendant violated such nonexistent prohibition. He could not be guilty of performing acts innocent when performed but culpable under a statute enacted thereafter.
Error is assigned to the failure to give the following proposed instruction:
There is no statutory requirement under the laws of the State of Washington that a physician examine a person before administering, prescribing, or selling drugs to said person.
You are further instructed that a prescription for drugs is not required. Drugs may be sold upon the written or oral order of a physician, surgeon, dentist or veterinary surgeon, duly licensed to practice in the State of Washington.
A trial court is not required to give negative instructions as to matters that will not support a conviction in a criminal case. It is unnecessary to explain those things which will not constitute a crime, though a court may do so in the interest of clarity. State v. Brooks, 73 Wn.2d 653, 440 P.2d 199 (1968); State v. Harvey, 57 Wn.2d 295, 356 P.2d 726 (1960); State v. Hannigan, 3 Wn. App. 529, 475 P.2d 886 (1970). If this proposed instruction had been given, it would have afforded further enlightenment to the jury and afforded assistance to the defendant in clearly presenting his theory of the case; however, it did not constitute prejudicial error to refuse it.
*676The state introduced evidence that one of the defendant’s assistants previously had sold drugs illegally in his office. This evidence could be relevant to show the absence of accident or mistake or a common scheme or plan if participation by the defendant in these acts was shown and was necessary to prove ¡an essential element of the crime charged. State v. Goebel, 40 Wn.2d 18, 240 P.2d 251 (1952). Since a retrial is necessary, the trial court will be in the preferable position at that time to evaluate whether the evidence then proffered meets the criteria for the admission of evidence of this kind as specified by State v. Goebel, supra.
It was not error to permit the criminologist from the laboratory of the police department to testify to the medical use of the drug for diet control. The decision as to the qualifications of a witness to testify as to a particular matter is within the sound discretion of the trial court. There was no abuse of that discretion. State v. Tatum, 58 Wn.2d 73, 360 P.2d 754 (1961); State v. Stringer, 4 Wn. App. 485, 481 P.2d 910 (1971); 2 J. Wigmore, Evidence § 555 (3d ed. 1940).
Assignments of error are raised which challenge the trial court’s denial of motions made at the completion of all testimony. The evidence was sufficient to present the case to the jury for decision.
The judgment is reversed, and the cause remanded for further proceedings consistent with this opinion.
Horowitz, C.J., concurs.