The Crown must be shown that the items in evidence before the court in trial were the same items seized during the investigation.
However, it has been said that generally speaking, "continuity of an exhibit goes to weight, not to admissibility." A party who cannot "prove absolute continuous possession...would not preclude admissibility".
There is no specific requirement to that the crown lead continuity evidence. Nor does every person in the chain of possession need to testify. The evidence of continuity should be lead where there is potential evidence before the court, by direct or circumstantial evidence or by inference, that may raise doubt as to the continuity. Even where gaps may raise a reasonable doubt about the item in court is the same seized initially by police, it may still be admissible and the doubt will simply go do weight.  For example, problems with continuity and integrity of recordings generally goes to their weight not the admissibility of the contents of the recordings.
Where there is conflicting evidence of the source of exhibit, it is up to the trier of fact to determine. It will therefore still be admitted into evidence.
The evidence seized by police will be cataloged and given identifying numbers. These numbers should remain consistent throughout the case. So whether the items are sent to a lab for analysis, they will always be identifiable as the same once they are presented in court. Any changes of the numbering system should be recorded and explained in court. Failure to properly track the exhibits by their identifying numbers may raise doubt as to their continuity.
R.v. Donald (1958), 121 CCC 304 (NBCA)(*no link)
R.v. Oracheski (1979), 1979 ALTASCAD 140 (CanLII), 48 CCC (2d) 217 (Alta SCAD)
R.v. De Graaf (1981), 1981 CanLII 343 (BC CA), 60 CCC (2d) 315
R.v. Andrade (1985), 18 CCC (3d) 41 (Ont CA)(*no link)
- R v West, 2010 NSCA 16 (CanLII) at para 130
see also R v Krole, 1975 CarswellMan 119(*no link) at para 27
- Krole at para 27
- R v Adam, 2006 BCSC 1430 (CanLII), (2007) BCWLD 1987 at page 7
R v Adam at page 7
R v Andrade
- R v Meer,  A.J. No. 1123 (Q.B.), 2010 ABQB 617 (CanLII) at para 16
R v Penney, 2000 CanLII 28396 (NL SCTD) at para 45
Ewaschuk, Criminal Pleadings and Practice in Canada the author states at p. 16-61
R. v Andrade at pp. 60-63
- e.g. R v Martin, 2008 ONCJ 601 (CanLII) at para 16
The Crown must prove that the drugs presented in court are the same that were seized at the scene of the investigation. Where the proof of the item is part of an essential element of the case, such as in a drug possession case, then it should be proven beyond a reasonable doubt. Gaps in continuity are not fatal unless they raise a reasonable doubt about the exhibit’s integrity.
In drug cases, where evidence does not establish continuity before being sent to the lab, a doubt arises and must be resolved in favour of the accused.
Under s. 53 of the CDSA, the continuity of exhibits can be proven by affidavit:
Continuity of possession
53. (1) In any proceeding under this Act or the regulations, continuity of possession of any exhibit tendered as evidence in that proceeding may be proved by the testimony of, or the affidavit or solemn declaration of, the person claiming to have had it in their possession.
Alternative method of proof
(2) Where an affidavit or solemn declaration is offered in proof of continuity of possession under subsection (1), the court may require the affiant or declarant to appear before it for examination or cross-examination in respect of the issue of continuity of possession.
Establishing a fact by way of exclusive opportunity requires proof that can take the form of continuity. 
- e.g. R v Panrucker, 2013 BCCA 137 (CanLII) - acquitted because no continuity evidence of access to the accused's cell where drugs were found.