In March, 2008, after a jury trial in the District Court, the defendants, brothers Ronald Mendes and Raymond Mendes,2 were convicted of several violations of the controlled substances laws.3 On appeal, they claim that the admission of certificates of drug analysis to prove the charges against them was constitutional error that was not harmless beyond a reasonable doubt. They also claim that their motions to suppress evidence found during the execution of a search warrant at their apartment should have been allowed; that expert testimony from a police witness exceeded permissible bounds; and that testimony concerning telephone calls placed to their cellular telephones and heard by police should not have been admitted.
While none of the other issues has merit, we conclude that the admission of certificates of drug analysis in violation of the defendants’ rights of confrontation requires that their convictions be reversed.4 Based upon our understanding of the relevant cases, we reach this conclusion even though both defendants *476testified at trial and, during that testimony, made admissions consistent with their defense that they were drug users, but not drug dealers. In this respect, we differ from our dissenting colleague.
1. Background, a. The Commonwealth’s case. The Commonwealth adduced evidence that, on October 21, 2006, Detective James Hyde of the Somerville police department, along with several other police officers, executed a search warrant at the defendants’ second-floor apartment at 98 Albion Street, a three-unit residential building located in Somerville, across the street from a public playground. When the police entered the apartment, Raymond was found near the rear bedroom of the apartment, and his brother Ronald was found in a different bedroom with his girlfriend.
The police presented the search warrant and advised both Raymond and Ronald of their Miranda rights. The defendants initially denied that any drugs were in the apartment, but Raymond later admitted that there were “some trees” — street slang for marijuana — in his bedroom.
Detective Hyde oversaw the search of the apartment. At trial, he and Detective Dominic Pefine, another member of the search team, testified as to the results. In Raymond’s bedroom, police found a clear plastic bag containing .46 grams of what was assumed to be cocaine and two pills that were identified as ecstasy, in a vase on top of the dresser; $740 in currency inside the dresser; and a black box under the bed that contained a bag of what was assumed to be marijuana, as well as $420 in currency, a 100-gram weight of a type used with a triple-beam scale, and a cellular telephone. In Ronald’s bedroom, police found a plastic bag containing 1.46 grams of what was assumed to be cocaine on the bureau; twelve bags of what was assumed to be marijuana in the pocket of a shirt in the closet; $943 in currency in the closet; $158 in currency on top of a television set; and a cellular telephone. In the living room, police found a brown leather jacket containing two bags of what was assumed to be marijuana; various papers belonging to the defendants; a plastic baggie with the comer ripped off; three cellular telephones; and two notebooks containing lists of names and dollar amounts.
Detective Hyde stated that he was familiar from his training *477and experience with both cocaine and marijuana and how these substances are ingested. Detective Pefine also testified that he was familiar with marijuana. Both officers described the general physical appearance of marijuana as a green leafy herbal substance. Through the testimony of Detective Hyde, the drugs seized were identified and admitted in evidence along with seven corresponding certificates of drug analysis prepared by the State laboratory.
While the search warrant was being executed, Detective Hyde monitored incoming telephone activity on the defendants’ cellular telephones. Approximately ten to twelve calls came in during this period. According to Detective Hyde, all of the callers alluded to purchasing drugs, several calls were very brief, and most callers did not identify themselves.
Detective Hyde described two calls in detail. One was from an individual who identified himself as Ed and asked to purchase $100 worth of cocaine. Detective Hyde informed him that he could fill the order and directed Ed to the intersection of Albion and Lowell Streets. When Ed reached that location, he placed a second telephone call to Detective Hyde, who sent a marked cruiser over. As the cruiser approached, Ed called again, saying that he should not be met at that intersection because the police were there.
The second call was from a woman who also sought to buy drugs. She asked for Ray or Ron and then asked to purchase cocaine for herself and marijuana for her niece. Detective Hyde said that he could satisfy her request and directed her to a nearby liquor store. Again, he sent a marked cruiser to the designated location. The woman later called and indicated that she had been stopped by the police, but still wanted to purchase drugs.
Detective Sergeant David Montana, the head of the Medford police department’s drug-control unit, testified as an expert witness for the Commonwealth. Among other things, he explained what cocaine and marijuana look like, the forms they may take, and how they are ingested and packaged for sale. He stated that he “believe[dj” one substance in evidence was “about half a gram” of cocaine and that another substance “appealed] to be marijuana.” In response to a hypothetical question, it was his *478opinion that the summary of the evidence put to him was not consistent with personal use.
b. The defendants’ case. The theory of the defense was that the brothers were drug users, but not drug dealers, and that any drugs found in their apartment were for their own use. Ronald introduced the testimony of a friend who stated, among other things, that he would smoke marijuana with Ronald. Raymond introduced the expert testimony of Dr. Alan Wartenberg, a physician who specializes in the treatment of addiction and who described the consumption and purchasing habits of heavy drug users. In addition, each defendant testified on his own behalf.
Raymond testified that the records found by the police were for the purpose of keeping track of money that the defendants’ friends had donated to help them buy music studio time for Raymond’s son, an aspiring musician. He also offered benign explanations for the presence of the significant amounts of cash found in the apartment. As for his drug use, Raymond said that he would purchase an ounce of marijuana each week and that he would roll the marijuana into a “blunt” cigar, sprinkle cocaine on top, and smoke it. He stated that he kept “the drugs” in the places where Detective Hyde said he found them in order to hide them from Ronald’s children, who would come to visit. He also explained that he took ecstasy pills as a sex drug when “a lady friend” came to visit.
Ronald testified that he smoked marijuana every day, three to four times per day, and that he also used cocaine approximately three times per week. Like Raymond, he offered innocent explanations for the presence of cash in the apartment. He admitted that the bags of marijuana found in the pocket of the shirt in his closet and in the jacket in the living room were his, explaining that he had purchased marijuana for his personal use from a dealer who only sold it in “dime bags,” rather than in larger quantities. He also acknowledged that the cocaine found in his room was his.
2. Certificates of drug analysis. The Commonwealth had the burden to prove beyond a reasonable doubt that the substances seized from the defendants’ apartment actually were marijuana, cocaine, and ecstasy. Commonwealth v. Vasquez, 456 Mass. 350, 361 (2010). As held by the United States Supreme Court in *479Melendez-Diaz v. Massachusetts, 129 S. Ct. 2527 (2009), the admission of the certificates of drug analysis to meet the Commonwealth’s burden, without affording the defendants the opportunity to cross-examine the chemical analyst who prepared them, violated the defendants’ rights under the confrontation clause of the Sixth Amendment to the United States Constitution. Although the defendants did not object, on confrontation grounds, to the admission of the certificates at trial, because their case was tried after the Supreme Judicial Court’s decision in Commonwealth v. Verde, 444 Mass. 279 (2005), and before the issuance of Melendez-Diaz, they are entitled to have this constitutional error reviewed under the “harmless beyond a reasonable doubt” standard. Vasquez, supra, at 356-360.
“[T]o establish harmlessness beyond a reasonable doubt, the Commonwealth must show that other properly admitted evidence of guilt is ‘overwhelming,’ in the sense that it is ‘so powerful as to “nullify any effect’” that the improperly admitted evidence ‘might have had’ on the fact finder or the findings.” Id. at 362, quoting from Commonwealth v. Tyree, 455 Mass. 676, 704 n.44 (2010). “We pay particular attention to whether the Commonwealth’s case ‘radiates from a core of tainted evidence.’ ” Commonwealth v. Fluellen, 456 Mass. 517, 526 (2010), quoting Tyree, supra at 702. “[Wjhere the improperly admitted evidence directly implicates the Commonwealth’s proof of an element of the crime, our inquiry must necessarily focus on the effect of the improperly admitted evidence as it pertains to that element.” Vasquez, supra at 362.
In this case, although the Commonwealth introduced abundant properly admitted evidence of drug dealing, this evidence did not establish that the substances seized actually were cocaine, marijuana, and ecstasy as charged in the criminal complaints against the defendants. See Vasquez, supra at 366. See also Commonwealth v. Morales, 76 Mass. App. Ct. 663, 668 (2010). Other than the certificates of drug analysis, the Commonwealth’s evidence bearing in any respect on the nature of the substances consisted only of general descriptions by the police witnesses of the typical appearance of cocaine and marijuana, the conclusory use by witnesses of the words “cocaine” and “marijuana” to describe the substances found, the testimony of Detective *480Montana that he “believe[dj” one substance was “about half a gram” of cocaine and that the other substance “appeared] to be marijuana,” and Raymond’s statement to Detective Hyde that there were “some trees” in his bedroom. This evidence is comparable to, and no stronger than, evidence in other cases where it has been held that the potent evidentiary effect of erroneously admitted certificates was not nullified. See Vasquez, supra at 365; Commonwealth v. Rivera, 76 Mass. App. Ct. 67, 69-70 (2009), Commonwealth v. Melendez-Diaz, 76 Mass. App. Ct. 229, 233 (2010); Commonwealth v. Pimentel, 76 Mass. App. Ct. 236, 239 (2010); Commonwealth v. Rivas, 77 Mass. App. Ct. 210, 212 (2010); Commonwealth v. Hernandez, 77 Mass. App. Ct. 259, 263 (2010); Commonwealth v. Barbosa, 77 Mass. App. Ct. 340, 344-345 (2010).
The Commonwealth maintains, however, that we also must take into account evidence admitted during the defendants’ case — in particular, the defendants’ own testimony concerning their drug use.5 We are unconvinced that it is appropriate for us to do so. The Supreme Judicial Court has indicated on more than one occasion that, short of a stipulation as to the nature of the drugs — which, by express agreement, conclusively establishes the facts contained therein — the Commonwealth is not relieved of its burden of proving the composition of the substances in question with properly admitted evidence. Commonwealth v. Vasquez, supra at 361, 368. This is so even where the defense strategy is to concede possession and contest only the defendant’s intent to distribute, and even where the defendant has conveyed through words or conduct that the items in question are drugs. See Commonwealth v. Charles, 456 Mass. 378, 383 (2010); Commonwealth v. Fluellen, supra at 526-527.6
We disagree with the dissent that admissions coming in the *481form of the defendants’ sworn testimony at trial stand on a special footing and may form the basis for concluding that the admission of the certificates was harmless. In this regard, the case of Commonwealth v. Charros, 443 Mass. 752, cert, denied, 546 U.S. 870 (2005), is instructive. In Charros, one of the issues presented was whether the erroneous admission of incriminating evidence that should have been suppressed (money found in the defendant’s purse when she was illegally seized by police) was harmless beyond a reasonable doubt. In confronting that issue, the Charros court was required to consider the significance of the defendant’s own trial testimony explaining and thereby admitting possession of the erroneously admitted evidence. After noting that there were conflicting views among courts and commentators on the point, the court rejected the Commonwealth’s argument that the defendant’s testimony rendered the error harmless beyond a reasonable doubt, observing that “[i]t is virtually impossible to say, on this record, whether [the defendant] would have testified had the evidence been suppressed.” Id. at 766. Similarly, in the present case, we cannot know whether the defendants’ testimony was a response to the erroneous admission of the certificates and hence tainted by the error. We therefore conclude that we should not consider their testimony in analyzing the question of harmlessness, lest we compound the prejudicial effect of the certificates’ admission.
Unlike our dissenting colleague, we see no determinative distinction between this case and Charros. No less than in Char-ros, the defendants’ testimony went to a potentially outcome-determinative issue; and, no less than in Charros, the defense strategy, including the defendants’ decision to testify and any concessions made by counsel, was based upon the hand they were dealt. The defendants may well have made choices in light of the existing state of the law in Massachusetts, which allowed the Commonwealth to use certificates of drug analysis, without more, to meet its burden of proof as to the nature of the substances *482forming the basis of the charges. See Vasquez, supra at 359-360, & 368 n.21.
In any event, even if we were to take the defendants’ testimony into account, we would not be assured that the impact of the drug certificates was nullified. While they admitted to being substantial users of drugs, and used the nomenclature “marijuana” and “cocaine” during their testimony, the defendants made no direct admissions about the composition of the charged substances. Compare Commonwealth v. Villatoro, 76 Mass. App. Ct. 645, 652-653 & n. 11 (2010).7 In short, when the issue properly is framed as whether there was overwhelming evidence, apart from the erroneously admitted certificates, of the actual nature of the substances forming the basis of the charges, we must conclude that there was not, whether or not the defendants’ evidence is included in the analysis.
3. Other issues. The defendants’ remaining arguments require little discussion.
a. Search warrant. There is no merit to the claim that the affidavit submitted by Detective Hyde in support of the application for a search warrant failed to establish the veracity of the two confidential informants referenced therein, and, hence, did not supply probable cause. See generally Commonwealth v. Upton, 394 Mass. 363, 369-377 (1985) (adopting Aguilar-Spinelli8 standard for purposes of art. 14 of the Massachusetts Declara*483tion of Rights). The affidavit established that the identity and places of residence of both informants were known to Detective Hyde, see Commonwealth v. Welch, 420 Mass. 646, 651 (1995); that the information provided by both informants was extremely detailed, see Commonwealth v. Alfonso A., 438 Mass. 372, 376 (2003); that the informants corroborated each other; and that the information they supplied was further corroborated by police investigation and observation. Most significantly, the second informant engaged in two controlled buys, both within seven days of the application for the search warrant. See Commonwealth v. O’Day, 440 Mass. 296, 301 (2003).
Contrary to Raymond’s position, it was not necessary for Detective Hyde to have personally observed the actual controlled buys. See Commonwealth v. Luthy, 69 Mass. App. Ct. 102, 106-108 (2007). Furthermore, the magistrate was entitled to conclude, for purposes of issuing the search warrant, that the substance purchased by the informant was, in fact, cocaine. See Commonwealth v. Byfield, 413 Mass. 426, 430 (1992).
Ronald’s additional contention that there was an insufficient nexus between any drug activity and the brothers’ apartment is also groundless. In addition to two controlled buys made by the second informant, where the defendants were observed leaving their apartment and traveling directly to the prearranged buy location close to their residence, the first informant directly observed more than twenty cocaine and marijuana sales in the apartment and provided detailed information about where drugs and paraphernalia were kept at that location. Contrast Commonwealth v. Pina, 453 Mass. 438, 442 (2009).
b. Expert opinion. Detective Montana’s opinion testimony did not exceed appropriate bounds. He provided general, educational information for the jury about the significance of certain facts and whether they were consistent with drug distribution rather than personal use. A hypothetical question posed to him, which closely reflected the trial evidence, was not impermissible. See Commonwealth v. Tanner, 45 Mass. App. Ct. 576, 579 (1998).
c. Calls to defendants’ cellular telephones. For the first time on appeal, the defendants contend that Detective Hyde’s testimony concerning calls placed to the defendants’ cellular *484telephones was inadmissible hearsay and violated their rights of confrontation.9 It is evident, however, that the testimony was not offered for the truth of the statements made by the callers, but, rather, was offered for the nonhearsay purpose of establishing that the telephones were instrumentalities employed by the defendants to facilitate the sale of drugs. The defendants’ intent to distribute was the core issue in the case, and the prosecutor used the testimony concerning the telephone calls to argue that the defendants had organized and equipped themselves to sell, and not merely to use, drugs. Because the testimony was not hearsay, the confrontation clause was not implicated. See Crawford v. Washington, 541 U.S. 36, 59 n.9 (2004); Commonwealth v. Brum, 438 Mass. 103, 116 (2002).
The defendants’ alternative argument that the judge was required sua sponte to give a limiting instruction when this testimony was received is without merit. See Commonwealth v. Washington, 449 Mass. 476, 488 (2007).
4. Conclusion. Because we are unable to determine that the admission of certificates of drug analysis played little or no role in the outcome of the case, we conclude that this constitutional error was not harmless beyond a reasonable doubt. Accordingly, the defendants’ convictions are reversed, and the verdicts set aside.
So ordered.
Because the defendants are brothers who share the same surname, we will refer to them by their first names.
Both Ronald and Raymond were convicted of possession of a class B substance (cocaine) with intent to distribute, in violation of G. L. c. 94C, § 32A(a); possession of a class D substance (marijuana) with intent to distribute, in violation of G. L. c. 94C, § 32C(a); and a drug violation in a school or park zone, in violation of G. L. c. 94C, § 32J. Raymond was also convicted of possession of a class B substance (ecstasy), in violation of G. L. c. 94C, § 34. The convictions of possession of marijuana with intent to distribute and Raymond’s conviction of possession of ecstasy were placed on file. Prior to trial, conspiracy charges against both defendants were dismissed at the Commonwealth’s request.
Although we ordinarily do not consider appeals from indictments (or counts of a complaint) that have been placed on file, “we may do so, in our discretion, in a suitable case.” Commonwealth v. Spearin, 446 Mass. 599, 606 (2006). We consider this to be a suitable case because the counts of the complaints that were placed on file, see note 3, supra, present issues identical to those raised by the convictions from which the appeals were taken.
The Commonwealth also relies upon the testimony of Dr. Wartenberg, who described the contents of one of the exhibits as “a white powder in a plastic bag inside of a plastic bag that is consistent with cocaine.” Again, in conformity with Vasquez, supra, and the Appeals Court cases cited above, this testimony is inconclusive and of minimal value in proving the actual composition of the substance.
The recent case of Commonwealth v. DeMatos, 77 Mass. App. Ct. 727, 731-732 (2010), does not detract from this analysis. In DeMatos, it was the Commonwealth, during its case-in-chief, that introduced admissions made by the defendant to the police when they entered his apartment. These admis*481sions, as well as the observations of the police witnesses, established, among other things, that the defendant had been ingesting cocaine when the police arrived and was exhibiting the effects of drugs apparently from the same supply that was seized during the police search. In these circumstances, “the evidence that the composition of the drugs in the apartment was cocaine was so powerful that the certificates had little or no effect on the verdicts.” Id. at 732.
Our dissenting colleague places emphasis on Villatoro to support her position. The factual situation presented in Villatoro was, however, quite different. Unlike this case, Villatoro involved only one defendant who was charged with possession of only one drug, marijuana, with the intent to distribute. A police officer, implicitly found by the judge to be qualified, testified that he “recognized the odor of marijuana when he approached the defendant based on (the officer’s) own experience, including hundreds of marijuana arrests.” Id. at 654. Therefore, in addition to the defendant’s testimony, there was powerful expert testimony of physical observations identifying the nature of the substance.
Here, the counts of the complaint charged that Ronald possessed two illegal substances, cocaine and marijuana, with the intent to distribute, while Raymond was charged with those same crimes as well as possession of ecstasy. Seven different certificates of analysis were admitted at trial. For the defendants to be convicted of the five counts pertaining to drug possession, it was necessary to prove that the illegal substance specified in each count was, indeed, that substance.
Aguilar v. Texas, 378 U.S. 108 (1964). Spinelli v. United States, 393 U.S. 410 (1969).
At oral argument, we requested further briefing as to whether listening to the calls violated G. L. c. 272, § 99. After review of the postargument submissions, we are persuaded that there was no unlawful electronic interception within the definition of G. L. c. 272, § 99.