Common Design Control Mistakes in Combination Product Development
15 Sep 2026
A combination product can have a technically strong device, a well-characterized drug and a carefully prepared development plan, yet still experience problems because the design-control process was not properly connected.
This is true when the product will be used by patients or other non-professional users. A device may perform correctly in laboratory testing, but if users cannot understand how to assemble it, select the dose, administer the drug or recognize completion of the dose, the product may still create a safety risk.
The challenge is therefore not only to develop a product that works, but to develop one that works safely and predictably in the hands of its intended users.
A useful human-factors principle is that manufacturers should anticipate use errors and, where possible, design the product so that the error is eliminated or made unlikely rather than relying primarily on instructions and training.
What Are Common Design Control Mistakes?
Design control mistakes are not always caused by a lack of technical knowledge. In many cases, they occur because development activities are performed separately rather than as one connected process.
The following are some common problems that can affect combination-product development.
1. Treating the Device and Drug Separately
One of the first mistakes is developing the drug and device as though they will never interact.
A device may work correctly during engineering testing, but its performance can change when it is combined with the actual drug formulation, container or delivery configuration.
For example, injection force may be affected by formulation characteristics. Similarly, the way a user interacts with the device may influence whether the correct dose is delivered.
Design controls should therefore consider the complete combination product, including important interfaces between the constituent parts.
2. Starting With the Design Instead of User Needs
Another common mistake is moving too quickly into engineering specifications without sufficiently understanding who will use the product and where it will be used.
Combination products such as autoinjectors, inhalers, nebulizers and drug patches may be used by patients in their homes. These users may have limited knowledge of medical terminology or drug-delivery techniques. Expert identifies several common use errors associated with such products, including incorrect dose selection, incorrect administration technique and failure to complete the delivery process.
The design process should therefore begin with intended users, intended use and use environment. A technically elegant product is not necessarily a usable product.
3. Blaming the User for Use Errors
When a user selects the wrong dose or performs a task incorrectly, it can be tempting to record the cause as “user error.” This approach can hide the actual design problem.
For example, if a patient selects the wrong dose because the dose-selection display is difficult to understand, the more useful question is whether the interface clearly communicated the selected dose. Our consultants recommends looking at the interaction between the user and the user interface rather than simply blaming the user. The distinction is important because the objective is to identify the underlying cause and improve the design. The practical question should be:
What could be changed in the product to make the correct action easier and the incorrect action less likely?
4. Relying Too Much on Instructions and Training
Instructions for use, warnings and training are important, but they should not automatically become the primary solution to every use-related problem.
If a user can accidentally assemble a device incorrectly, simply adding another paragraph to the instructions may not be the best solution.
The preferred approach is generally to address the problem through the design itself where reasonably possible.
Examples described include designing components so they can only be assembled one way, using alignment cues, providing tactile or audible feedback and preventing operation when assembly is incorrect. Instructions and training can then provide additional support rather than carrying the entire safety burden.
5. Identifying Use Errors Too Late
Waiting until the final validation study to discover usability problems is another significant mistake.
Human-factors evaluation should be iterative. Formative evaluations with representative users can identify problems while the design is still being developed.
The general process is:
Understand users → Develop design → Build prototype → Conduct formative evaluation → Improve design → Re-evaluate → Validate
Our cosnultants described this as an iterative HFE/UX process, with findings from formative testing feeding back into design improvements. This approach is much more practical than discovering major interaction problems when the design is already close to commercialization.
6. Failing to Maintain a Living Use-Related Risk Analysis
A use-related risk analysis should not be considered a document that is completed once and placed in the design history file.
As new use errors are identified, the risk analysis should be updated. When a mitigation is introduced, the effect of that mitigation should also be evaluated.
For example, increasing the size of important information in an instruction may make that information easier to see, but could also reduce the visibility of another important instruction. The risk analysis therefore needs to evolve with the product.
7. Poor Root Cause Analysis
Finding a use error is only the beginning. The development team must determine why the error occurred. Was the problem caused by the physical design? The user interface? The task? The environment? The instructions? Or a combination of these factors?
A good root cause analysis may involve observing the interaction, interviewing users, reviewing the prototype and examining the design itself. Without understanding the root cause, the team may implement a corrective action that treats the symptom but leaves the underlying problem.
8. Not Re-evaluating the Design After Changes
A design change intended to solve one problem can create another.
Suppose a warning is made more prominent in the instructions. If the new presentation makes another critical instruction less noticeable, the original problem may have been reduced while a new one has been introduced.
This is why design solutions should be evaluated again after implementation.
Experts specifically recommends re-evaluating the design to determine whether mitigations are effective and whether they introduce new interaction problems.
What Should a Better Design-Control Process Look Like?
A practical approach is to keep the development activities connected:
- Understand the Users: Identify intended users, use environments and important tasks.
- Define Requirements: Convert user needs into clear design and user-interface requirements.
- Identify Risks: Identify potential use errors and their associated harms.
- Design the Solution: Address important risks through the product design wherever possible.
- Evaluate Early: Use formative testing to identify problems while changes are still manageable.
- Update the Risk Analysis: Document identified errors, causes and risk-reduction measures.
- Verify the Design: Confirm that design outputs meet specified requirements.
- Validate With Representative Users: Confirm that the final product can be used safely and effectively for its intended purpose.
This creates a development cycle rather than a series of disconnected documents.
How TS Quality & Engineering Can Help
The most common design-control mistakes in combination-product development often come from disconnection: between the drug and device, between user needs and design requirements, between risk analysis and design decisions, or between formative findings and the final product.
Good design control should prevent these gaps. It should bring together user needs, design inputs, risk management, human factors, verification and validation throughout development.
For combination products, the objective should not be to design a product and then teach users how to overcome its weaknesses. The stronger approach is to understand foreseeable use errors and, wherever reasonably possible, address them through the design itself.
TS Quality & Engineering (TSQE) supports drug-device combination product development through practical expertise in design controls, user needs, design inputs, risk management, human factors, verification and validation, traceability and technical documentation. TSQE can help organizations identify weaknesses in their development process and establish a structured approach that connects design decisions with user needs, risks and objective evidence.
