The employer and insurer appeal from the compensation judge’s findings that the employee was not intoxicated or engaged in a prohibited act at time of his personal injury. We affirm.

In the fall of 2006, Jeremiah Ewer was twenty-seven years old, having worked as a roofer since he was nineteen and having drunk alcohol, smoked marijuana, and used cocaine since about the age of seventeen.^[1] Except for a two-year period of abstinence between 1998 and 2000, he has admitted to smoking marijuana about once or twice a week during that time on a regular basis and snorting cocaine about once or twice a year. On the evening of Monday September 25, 2006, Mr. Ewer, who had at that time been working as a roofer for about eight years, smoked two or three joints of marijuana, and the next day he went to work and did his job. That evening, September 26, 2006, he snorted three or four lines of cocaine and then went to bed at about 10:00 or 10:30 p.m.

On the morning of Wednesday, September 27, 2006, Mr. Ewer [the employee] awoke at about 5:30 or 6:00 a.m. and drove to the Anoka site of his job with Allweather Roof, Inc. [the employer], where he was to start work at 6:30 a.m. The employer was eligible at that time for a partial rebate from its workers’ compensation insurer for its good safety record, and prior to the start of the work shift on September 27, 2006, the employee’s foreman, Vincent Neumann, gathered his crew together to remind them that they should work in a safe manner. He specifically reminded all employees to stay away from the edge of the roof on which they would be working, a rope with safety flags having been strung that day six feet from that edge to mark off a controlled access zone.

The weather that day was apparently mixed, and it eventually included rain and hail as well as sunshine. At about 2:00 p.m. on September 27, 2006, while nailing strips of termination metal at the edge of the roof, the employee fell from the roof about thirty feet to the ground, sustaining injuries that required immediate hospitalization. At the time of his fall, the employee had been working outside the safety-flagged rope, inside the six-foot controlled access zone, and he was not using a safety harness or a safety monitor. He was taken immediately for treatment to a hospital, where he was treated for a left hip/femur fracture and L4 compression fracture. At 3:50 a.m. the following day, about fourteen hours after the employee’s accident and injury, a specimen of the employee’s urine tested positive for cocaine and marijuana metabolites. About a month later, on October 24, 2006, the employer and insurer, who had initially admitted liability and commenced payment of benefits, discontinued the employee’s benefits pursuant to Minnesota Statutes section 176.221, subdivision 1.^[2] A month later, on November 21, 2006, the employee filed an Objection to Discontinuance.

On March 16, 2007, the employee underwent an assessment at Regions Hospital Alcohol and Drug Abuse Program. The record does not contain any narrative report of that assessment, but an assessment summary from that facility indicates that the employee admitted to a history of alcohol, cocaine, and marijuana use. The assessment evaluator recommended that the employee was “to abstain from all mood altering chemicals, attend 12-step related meetings, and to attend a chemical health education seminar/class that will focus on alcohol and drug use and its relationship with legal, social, and physical issues and consequences.”

The employee’s claim for benefits came on for hearing before a compensation judge on March 28, 2007. Issues presented to the judge were as follows:

Testimony and exhibits offered into evidence at hearing, including the testimony of the employer’s operations manager and various co-employees of the employee, included evidence that the employer sponsors an annual training program for its employees each year, at which the duty to have fall protection is discussed. Also submitted was evidence that safety measures implemented by the employer at its work sites included the use of safety harnesses, the marking of controlled access zones, and the use of human “safety monitors” posted near co-employees who are working in dangerous locations.

Foreman Neumann also testified at hearing, in part that he did not specifically advise his crew to wear their safety harnesses on the date of the employee’s injury, because he believed that his warning that they stay away from the edge was sufficient. He testified also that he did not recall telling the employees to do any edging that day. He testified further that, because it had rained earlier that day and the crew was therefore already a little wet, the employer was getting ready to wrap things up for the day at the time the employee fell. Only the employee testified that it was actually raining at the time he fell from the roof. He testified further that he had not used marijuana on the day of the accident, and no testimony was offered suggesting that the employee was slurring his speech, losing his balance, or otherwise acting intoxicated.

Also testifying at hearing, for the employee, was toxicologist Thomas Burr. Mr. Burr made clear initially that he had reviewed both the employee’s medical records, including the toxicology test done on his urine on the morning of September 28, 2006, and the March 27, 2007, report of medical toxicologist Dr. Samuel Hall, who had performed a medical record review at the request of the employer and insurer. He testified further, however, that he was nevertheless of the opinion that neither the cocaine nor the marijuana metabolites found on the employee’s toxicology test had any impairing effect at the time of the employee’s fall. In his opinion, Mr. Burr testified, smoking marijuana on Monday would not continue to have an impairing effect on Wednesday. He testified that the toxicology test done on the employee on the morning after his fall was evidence that the employee had used those substances but was not evidence sufficient to establish intoxication.

Also submitted into evidence by the employer and insurer, post-trial, was a video-taped deposition of Dr. Hall, taken April 26, 2007. Dr. Hall testified that, in his opinion, the employee was intoxicated on September 27, 2006, when he fell off the roof, that chronic marijuana intoxication altered his judgment and cognitive function, resulting in a disregard for safe work practices, and that his intoxication was the proximate cause of the injuries that he sustained.

The record closed on April 30, 2007. In a Findings and Order issued May 11, 2007, the compensation judge determined that the employee was not intoxicated at the time of his September 27, 2006, injury, nor was he barred from receiving benefits under the prohibited act doctrine. The employer and insurer appeal.

On appeal, the Workers' Compensation Court of Appeals must determine whether “the findings of fact and order [are] clearly erroneous and unsupported by substantial evidence in view of the entire record as submitted.” Minn. Stat. § 176.421, subd. 1 (2006). Substantial evidence supports the findings if, in the context of the entire record, “they are supported by evidence that a reasonable mind might accept as adequate.” Hengemuhle v. Long Prairie Jaycees, 358 N.W.2d 54, 59, 37 W.C.D. 235, 239 (Minn. 1984). Where evidence conflicts or more than one inference may reasonably be drawn from the evidence, the findings are to be affirmed. Id. at 60, 37 W.C.D. at 240. Similarly, findings of fact should not be disturbed, even though the reviewing court might disagree with them, “unless they are clearly erroneous in the sense that they are manifestly contrary to the weight of the evidence or not reasonably supported by the evidence as a whole.” Northern States Power Co. v. Lyon Food Prods., Inc., 304 Minn. 196, 201, 229 N.W.2d 521, 524 (1975).

Pursuant to Minnesota Statutes section 176.021, subdivision 1,

On appeal, the employer and insurer argue that this court should reverse the compensation judge’s decision on the issue of intoxication, based on the undisputed medical testimony of Dr. Samuel Hall. The employer and insurer contend that Dr. Hall’s testimony as to the effects of chronic intoxication, caused by longtime marijuana and cocaine abuse, is a specific medical condition not subject to a workers’ compensation judge’s usual knowledge or experience. Therefore, they argue, because the judge cannot simply ignore the undisputed medical opinions of Dr. Hall on this issue, the judge’s decision on this issue should be reversed. We are not persuaded.

While a compensation judge may not ignore or “disregard” uncontroverted medical opinion, see Flansburg v. Giza, 284 Minn. 199, 201-02, 169 N.W.2d 744, 746, 25 W.C.D. 3, 6 (1969), such opinion is not necessarily conclusive upon the trier of fact, see Tuomela v. Reserve Mining Co., 299 Minn. 203, 204, 216 N.W.2d 638, 639, 27 W.C.D. 312, 313 (1974), particularly where, as here, there is other evidence to consider. As the judge noted in his memorandum, even if intoxication was proven, there remains a significant issue as to proximate cause. As to that issue, the judge concluded, “What happened on the day in issue is that this experienced roofer, with a good work record and no history of prior injury, slipped due to weather conditions and hurrying on the job as opposed to chronic marijuana use.” It is evident that the judge found the employee’s testimony about the cause of his fall to be credible, and assessment of a witness’s credibility is the unique function of the trier of fact. Brennan v. Joseph G. Brennan, M.D., 425 N.W.2d 837, 839-40, 41 W.C.D. 79, 82 (Minn. 1988).

While evidently rejecting the opinion of Dr. Hall, the compensation judge accepted the testimony of Mr. Burr, that the test results of September 28, 2006, did not support a finding of intoxication on September 27, 2006. Nor does Dr. Hall dispute this conclusion. Where Dr. Hall’s opinion differs from Mr. Burr’s is on the theory of chronic intoxication. Dr. Hall testified that chronic marijuana intoxication has cognitive, behavioral, and physical effects on the individual that are different and separate from the normal effects of intoxication. In this case, even if the judge accepted Dr. Hall’s opinion on that issue, he was not compelled to conclude that such chronic intoxication was the proximate cause of the employee’s injury. Here, it was not unreasonable for the judge to reject Dr. Hall’s testimony in view of the employee’s testimony that he slipped off the roof. As we have stated on many occasions, there is a difference between disregarding unopposed medical opinion and rejecting it on other grounds. See Clark v. Archer Daniels Midland, slip op. (W.C.C.A. Feb 14, 1994). The judge’s findings in this case reference Dr. Hall’s testimony in some detail. The judge considered Dr. Hall’s opinion and found it unpersuasive in view of the employee’s medical records, the toxicology test, and the employee’s testimony regarding his use of and the effect that cocaine and marijuana had upon him.

The effect of possible intoxication on the employee’s injuries is essentially a fact issue, not a legal one. In the present case, the compensation judge accepted the employee’s testimony that he slipped due to weather conditions, not due to any alleged or even real intoxication. Concluding that it was not unreasonable, we affirm the judge’s conclusion that the employee’s injuries were not consequent to intoxication. Hengemuhle, 358 N.W.2d at 59, 37 W.C.D. at 239.

The employer and insurer appeal also from the compensation judge’s finding that the employee’s injury did not result from the performance of a prohibited act. They argue that, at the beginning of the workday on September 27, 2006, the employee’s foreman expressly ordered all of the workers to stay away from the edge of the roof and that the employee deliberately defied this order. Because the employee violated his foreman’s directive, they argue, the employee’s violation took him outside the scope of his employment and his claim for benefits should be denied. We are not persuaded.

Under the prohibited act doctrine, an intentional violation of a simple order or prohibition by the employer may take the employee outside the scope of the employment. Otto v. Midwest of Cannon Falls, 59 W.C.D. 25 (W.C.C.A. 1999). In Bartley v. C-H Riding Stables, Inc., the supreme court stated,

Where an employer expressly prohibits the doing of a certain specific act, the disregard of which is not reasonably foreseeable to the employer, a violation thereof takes the employee outside the scope of his employment and injuries resulting therefrom are not compensable even though the act might be considered to be in furtherance of the employer’s business.

Bartley v. C.H Riding Stables, Inc., 296 Minn. 115, 120, 206 N.W.2d 660, 663, 26 W.C.D. 675, 679-80 (1973). Thus, an employee who is injured while engaged in conduct specifically prohibited by the employer may be excluded from coverage under the workers’ compensation act. However, “[i]n order for the exclusion to be found, there must be an unequivocal and specific prohibition which is clearly communicated to the employee and which has been enforced by the employer.” Gunderson v. Mac’s Landscaping Ctr., 65 W.C.D. 274, 280 (W.C.C.A. 2005).

There are prohibitions which limit the sphere of employment and prohibitions which only deal with conduct within the sphere of employment. A transgression of a prohibition of the latter class leaves the sphere of employment where it was, and consequently will not prevent the recovery of compensation. A transgression of the former class carries with it the result that the man has gone outside the sphere.

Otto v. Midwest of Cannon Falls, 59 W.C.D. 25, 34, 35 (W.C.C.A. 1999), citing Rautio v. International Harvester Co., 180 Minn. 400, 231 N.W. 214, 6 W.C.D. 213 (1936), citing Eugene Dietzen Co. v. Industrial Bd. of Ill., 297 Ill. 11, 116 N.E. 684 (Ill. 1917). In the present case, the judge implicitly concluded that he viewed the employee’s transgression of the employer’s safety rules and his foreman’s instruction that day as not the type of violation that would preclude one from benefits under the prohibited act doctrine. Because it was not unreasonable, we affirm the compensation judge’s conclusion on this issue, together with his award of benefits to the employee. Hengemuhle, 358 N.W.2d at 59, 37 W.C.D. at 239.