My Product Failed the Challenge Test: What to Do Next

The report arrives and the result reads fail. After months of development, a failed challenge test feels like a disaster, but it is one of the most common outcomes in cosmetic development and it is almost always fixable. This guide explains what a failed result really means, how to read the report and exactly what to do next so that you pass the second time.

First: what does a failed challenge test mean?

A challenge test contaminates your product with five reference microorganisms and counts how many survive over 28 days. A fail means that, at one or more sampling points, the microbial counts did not fall far enough to meet the criteria of the standard you chose. It does not mean the formula is dangerous today. It means the preservation system is not strong enough to protect the product once a consumer starts contaminating it, so the product cannot be released as it stands.

How to read the report

The report tells you far more than pass or fail. Look for:

• Which organism failed. Bacteria, yeast and mould respond to different preservatives. A mould-only failure points to a different fix than a bacteria failure.
• At which day. A product that fails on day 7 but recovers by day 28 behaves differently from one where counts rise again late in the test.
• The size of the gap. A near miss may need a small adjustment, while no reduction at all suggests the preservative is not working in that formula.
• Which criteria you tested against. Criteria A is the recommended level of efficacy, and criteria B is a lower level accepted only when A cannot be met for justified reasons.

Read also What Is a Cosmetic Challenge Test? Preservative Efficacy Testing Explained

Why products fail: the 7 usual suspects

1. Preservative level too low. The dose was chosen from a supplier datasheet and never confirmed in your own formula.
2. pH outside the working window. Many preservatives only work in a narrow pH range, and organic acids lose power quickly as pH rises.
3. Ingredient interactions. Some surfactants, oils and thickeners bind or inactivate preservatives, so less is available in the water phase where microbes live.
4. Preservative spectrum gaps. One preservative rarely covers bacteria, yeast and mould equally. Weak coverage of one group is a classic cause of failure.
5. Too much microbial food. Plant extracts, hydrolysed proteins, sugars and botanical oils feed microbes and raise the demand on the system.
6. Natural-only systems. Preservative systems built only from “natural” ingredients are often weaker and need more development.
7. Packaging. Open jars and wide openings expose the product to more contamination than airless pumps.

Your recovery plan, step by step

1. Diagnose. Use the report to identify which organism failed and when.
2. Check the pH. Compare your pH with the working range of your preservative and correct it if needed.
3. Review the preservative level. Raise it within the limits of Annex V of the Cosmetics Regulation. Never exceed the maximum allowed concentration.
4. Broaden the spectrum. Combine two or more preservatives, or add a booster such as a chelating agent or a multifunctional ingredient that supports the system.
5. Remove troublemakers. If an ingredient inactivates the preservative, replace it or compensate for it.
6. Consider the packaging. Airless pumps or tubes reduce contamination during use.
7. Retest the new formula. Any change means a new challenge test from day 0.

What not to do

• Do not exceed the legal limit. If a permitted preservative still fails at its maximum level, the formula must change.
• Do not retest the same formula hoping for luck. The result is reproducible, so unchanged formulas usually fail again.
• Do not launch on a fail. The safety assessor cannot sign off a product without adequate preservation, and the risk of contamination is real.
• Do not change several things without a record. Document each change so you know what fixed the problem.

Read also Challenge Test Cost: What Affects the Price

Can criteria B rescue a fail?

Both common standards define a lower set of criteria, called B, for cases where criteria A cannot be reached for justified reasons, for example because stronger preservation would raise the risk of adverse skin reactions. This is not a shortcut. The justification has to be documented, and it is the safety assessor who decides whether it is acceptable in your CPSR. If you are close to passing on A, improving the formula is almost always the better route.

How long and how much will a retest cost?

A retest takes the full 28 days of the method, plus sample transit and reporting, so plan roughly one more month.To avoid paying for several rounds, test two or three formula variants at the same time, for example with different preservative levels, and select the best. You can read more in our guide to challenge testing.

Special case: natural and organic formulas

Natural formulas fail more often because their ingredients feed microbes and the preservatives approved for natural certification are limited. If you target a natural or organic standard, build the preservation system early, test early, and allow time for more than one development round. Remember that “natural” does not lower the legal safety requirement.

Read also How Long Does a Challenge Test Take?

Checklist before you retest

• I know which organism failed and on which day.
• I have measured the pH of the finished product.
• The preservative level is within the Annex V limit.
• I have documented every change to the formula.
• The sample sent is the new final formula in its final packaging.

Three examples from the lab

• The pH problem. A face cream with a good preservative failed on bacteria. The pH was too high for the preservative to be effective. Adjusting the pH and retesting solved it with no change in dose.
• The mould problem. A botanical gel passed on bacteria but failed on mould. Adding a second preservative with antifungal strength closed the gap.
• The surfactant problem. A shampoo needed far more preservative than expected because a surfactant bound part of it. Switching the surfactant system reduced the dose needed.

These are illustrative cases, but they show why reading the report matters more than simply increasing the dose.

Working with your safety assessor

Share the report with your safety assessor before you reformulate. They can tell you whether a change will affect other parts of the CPSR, such as the toxicological assessment of a higher preservative level, and whether criteria B could be justified. A short conversation early saves a second wasted month.

Terms you will meet in the report

• Log reduction: the fall in microbial count on a logarithmic scale. A 3-log reduction means a thousand-fold drop.
• Inoculum: the known amount of microorganisms added to your product at the start.
• Neutraliser: a substance that stops the preservative from acting during counting, so that survivors are not hidden.
• No increase: counts must not rise above the level of the previous sampling point.

When to ask for expert help

If two rounds of adjustments have not worked, bring in a formulation chemist or your safety assessor. Persistent failures often point to a structural issue, such as an incompatible ingredient or a formula that is simply too hospitable for microbes, and an expert can fix it faster than trial and error. The cost of an hour of advice is small compared with another 28-day cycle.

Frequently asked questions

Is a failed challenge test the end of my product?

No. Most failures are solved by adjusting the pH, the preservative level or the combination of preservatives.

Can I use a different laboratory for the retest?

Yes, but use the same standard so that the results are comparable, and keep the original report in your file.

Do I have to tell anyone about a failed test?

A failed test on an unreleased formula is part of development. What matters is that the product placed on the market has a passing result in its final formula.

Key takeaways

• A fail means the preservation system is too weak, not that the project is over.
• Read the report: organism, day and size of the gap guide the fix.
• Adjust pH, preservative level, spectrum and packaging, within legal limits, then retest.
• Test variants in parallel to save a month.