In the Philippine legal landscape, particularly under the stringent regime of Republic Act No. 9165, otherwise known as the Comprehensive Dangerous Drugs Act of 2002, the integrity of the evidence is the bedrock of the prosecution's case. Given the severe penalties involved—often reaching life imprisonment—the judiciary applies a high standard of scrutiny to the manner in which evidence is obtained.

The dismissal of a drug case on the grounds of planted evidence is a safeguard against state overreach and the violation of constitutional rights.

1. The Constitutional Basis: Due Process and the Presumption of Innocence

Every criminal prosecution begins with the Presumption of Innocence (Article III, Section 14(2), 1987 Constitution). When evidence is planted, it constitutes a "frame-up," which is a direct violation of Due Process.

The "Frame-Up" Defense: While Philippine courts often view the defense of frame-up with disfavor (as it can be easily concocted), it gains significant legal weight when the prosecution fails to establish a "broken chain of custody" or when there are glaring inconsistencies in the testimonies of the arresting officers.

2. The Chain of Custody Rule (Section 21)

The most common legal ground for dismissal when evidence is suspected of being planted is the failure to comply with Section 21 of R.A. 9165. This section dictates the "Chain of Custody," which is the method used to ensure that the drugs seized at the scene are the exact same substances presented in court.

The Mandatory Requirements (The "Three-Witness" or "Two-Witness" Rule): Depending on when the incident occurred (before or after the 2014 amendment by R.A. 10640), the law requires the presence of specific witnesses during the inventory and photographing of the seized items:

The Accused or their representative/counsel.
An Elected Public Official.
A Representative from the National Prosecution Service (DOJ) OR the Media.

Grounds for Dismissal:

Physical Absence of Witnesses: If these witnesses are not present at the time of seizure or immediately after, the "integrity and evidentiary value" of the drugs are compromised.
Failure to Mark Evidence Immediately: If the police fail to mark the seized drugs at the place of arrest (or at the nearest police station if the situation is unsafe), the court may rule that there was an opportunity for the police to "plant" or substitute the evidence.

3. The "Object Evidence" vs. Testimonial Evidence

In drug cases, the corpus delicti (the body of the crime) is the dangerous drug itself.

If there is a gap in the movement of the drug—from the "seizing officer" to the "investigating officer" to the "forensic chemist"—the court cannot be certain the evidence wasn't tampered with.
Dismissal Ground: When the prosecution cannot account for every "link" in the chain, the doubt created is resolved in favor of the accused, leading to an acquittal.

4. Violation of Constitutional Rights (Sections 2 & 3, Art. III)

Evidence that is planted is usually obtained through an Illegal Search and Seizure.

Fruit of the Poisonous Tree: Under Article III, Section 3(2) of the Constitution, any evidence obtained in violation of the right against unreasonable searches and seizures is inadmissible for any purpose in any proceeding.
Warrantless Arrests: Most planted evidence cases involve "buy-bust" operations. If the court finds that no actual sale took place (a "sham" operation), the subsequent search is illegal, and the planted drugs are excluded as evidence.

5. Indicators of Planted Evidence in Court

Judges look for specific "red flags" that support a claim of planted evidence:

Inconsistent Testimonies: Discrepancies between the affidavits of the police and their oral testimony regarding where the drugs were found.
Impossibility of the Act: If the physical circumstances make it highly unlikely that the accused could have possessed the drugs (e.g., the accused was in a different location or the "buy-bust" happened in an area where such a transaction was physically impossible).
Lack of Pre-operation Coordination: Failure to coordinate with the PDEA or provide a "Pre-Operation Report" can cast doubt on the legitimacy of the sting operation.

6. Penalties for Planting Evidence

Under Section 29 of R.A. 9165, the act of planting evidence is a heinous crime. Any person found guilty of "planting" any dangerous drug, regardless of quantity, shall suffer the penalty of Death (now converted to Reclusion Perpetua or life imprisonment without parole under R.A. 9346).

Summary Table: Key Grounds for Dismissal

Ground	Legal Basis	Reasoning
Non-Compliance with Sec. 21	R.A. 9165 / R.A. 10640	Failure to follow the chain of custody creates a window for evidence planting.
Inadmissibility	"Fruit of the Poisonous Tree"	Evidence obtained via illegal search cannot be used in court.
Failure to Prove Corpus Delicti	Rules of Evidence	If the identity of the drug is in doubt, the crime is not proven.
Reasonable Doubt	Art. III, Sec. 14, Constitution	If the defense of "frame-up" is backed by proof of police irregularity, the accused must be acquitted.