DNA and Forensic Analysis in Drug Cases

The use of DNA and forensic analysis in criminal investigations has grown markedly over the quarter of this century, and in the process has become an invaluable tool for securing evidence and solving crimes. Often, when people think of DNA and forensic evidence, they think of murder investigations. This evidence, however, is not strictly confined to violent crimes, but also plays a definitive role in cases involving drugs.  

This blog examines the role of DNA and forensic analysis in drug investigations, how such evidence is treated by the courts, and the challenges and limitations such evidence can present.

At Olliers we have specialist expertise and experience in defending clients facing serious drug allegations. Our experience dates back several decades and we have seen dramatic changes in the investigation landscape, not just in the area of DNA evidence. Many of our lawyers are leaders in their field. Olliers is ranked as a top tier law firm by both the Legal 500 2025 and Chambers Guide 2025. We are a Times Best Law Firm 2025. We are the Manchester Legal Awards 2025 Crime Team of the Year retaining the title from 2024, an award we have won eight times since 2011.

What is DNA Evidence?

Deoxyribonucleic acid (DNA) is a molecule that carries the genetic instructions for all living things. DNA evidence is incredibly valuable for identification in forensics because everyone’s DNA is unique (except identical twins).

Through the process of collecting biological samples (such as blood, hair, skin cells or bodily fluids) from crime scenes, it is possible to create a “DNA profile”. This profile can then be compared to the DNA profile of suspects to establish a match.

The Police and Criminal Evidence Act 1984 outlines the circumstances where fingerprinting, impressions of footwear, intimate samples and other samples can be taken from a detainee. It also stipulates that DNA profiles can be included on the National DNA Database.

The national DNA database was set up in 1995. It is run by the Home Office and contains in excess of 6,000,000 profile records and DNA material from in excess of 250,000 crime scenes is held on the database.

How Do Scientists Use It?

Forensic scientists piece together traces left behind from sources of DNA to attempt to establish a link between a particular individuals DNA and the crime scene/evidence. Forensic scientists often analyse biological and physical traces collected from crime scenes including:

1. DNA 
2. Fingerprints 
3. Blood 
4. Semen 
5. Footwear markings 
6. Clothing fibres

In drug-related cases, fingerprint evidence is often particularly significant. It is also becoming more common for the police to forensically examine the data stored on or transmitted through mobile phones. The usefulness of such DNA and forensic analysis cannot be understated.

Collection of DNA Evidence

When collecting DNA evidence, the upmost care must be taken to avoid the possibility of contamination and to ensure the integrity of exhibits. Contamination occurs when DNA or other biological material is introduced to an exhibit. The Forensic Science Regulator has published good practice guidance to reduce the risk of contamination as far as practically possible. 

Use of DNA Evidence in Drug Cases

In drug cases, DNA evidence can play a significant role in establishing a link between evidence and a defendant. For example, the use of fingerprint analysis in the investigation of drug offences provides valuable information. As fingerprint patterns differ from person to person, the evidence can be quite probative in establishing a connection to the crime. Through the analysis of matched ridge characteristics, scientists are able to identify whether the fingerprints from a defendant correspond to that taken from evidence, such as snap bags or drug packaging.

Admissibility of DNA Evidence in Court

The admissibility of DNA evidence and any forensic analysis is largely governed by Rule 19 of the Criminal Procedure Rules, the Police and Criminal Evidence Act 1984, and case law.

Often, such evidence is introduced by way of expert evidence. This usually includes the expert providing an opinion on the basis of the analysis they have conducted. For example, fingerprint evidence would require an expert considering the two sets of fingerprints and coming to an opinion based off the comparison as to whether the print on the item relied upon came from the defendant.

When considering whether expert evidence is admissible, the court will often consider the pre-conditions of admissibility relating to the expert’s evidence, most recently reviewed in the case of Kevin Brecani v R [2021] EWCA Crim 731. The requirements are set out as follows: 

1. The evidence is relevant to a matter in issue in the proceedings; 
2. The witness is competent to give that opinion; and 
3. It is needed to provide the court with information likely to be outside the court’s own knowledge and experience. 

Subject to the relevant provisions being adhered to, fingerprint or other DNA evidence may be admissible as a matter of law if it tends to prove the guilt of a defendant.

Challenges and Limitations of DNA/Forensic Evidence  

While DNA and forensic evidence can carry significant probative value, the use of such evidence is not without its risks.

Often, defence representatives can challenge the admissibility of DNA evidence on various grounds, including contamination, improper handling, or issues with the DNA analysis process. It is also possible for the evidence to be excluded due to the non-compliance of statutory provisions, the unreliability of the analysis, or the credibility of the expert witness.

A further layer of difficulty can arise when the DNA is found on a moveable object or contains a mixed DNA profile. In such circumstances, the court will be tasked with considering other factors, such as:

1. Was the object the DNA was found upon associated with the offence?  
2. How readily movable was the object in question? 
3. Any geographical association between the offender and the offence.  
4. If the DNA profile is mixed, is the defendant’s DNA profile a major contributor to the overall DNA? 
5. Is it more or less likely that the DNA attributable to the defendant was deposited by primary or secondary transfer?

Conclusion 

DNA and forensic evidence can play a crucial role in drug offences. As DNA evidence cannot provide absolute proof of an involvement in a crime, the probative value of any DNA evidence depends on the quality of it, and the relationship it has to all of the other evidence in the case. It is therefore essential to have expert representation that recognises the limitations of such evidence and the avenues to explore and challenge it.

At Olliers Solicitors, we have extensive experience in handling cases involving DNA evidence. Our team of expert solicitors works with leading forensic scientists to scrutinise the evidence, identify potential issues, and build a robust defence strategy. If you are facing allegations involving DNA evidence, contact us today for expert legal advice and representation.