Thinking · Evidence

The cold chain ended at the clinic. The heat did not.

Abhijith Magal · 22 September 2026 · 9 min read

The cold chain conversation ends at the last cold point: the fridge in the district store, the vaccine carrier, the pharmacy. After that, most medicines are "store below" medicines, and the journey continues without anyone watching the temperature. A strip of tablets rides in the back of a vehicle, waits in a storeroom with a tin roof, then sits in a home through the hottest months of the year. If it degrades along the way, nothing on the shelf changes. It is still there. It is Available in every count a programme keeps, and it may no longer do what it was prescribed to do.

That sharpens one of the four climate-access conditions in a way nothing else does. The medicine was delivered. The access still failed.

What do storage labels and stability testing actually promise?

That the product stays within specification through its shelf life under the long-term conditions it was tested at. WHO's guidance on stability testing, Annex 2 of Technical Report Series 953, ties those conditions to the climatic zone where a product will be marketed. The WHO Expert Committee split the hot zone in two in 2005: Zone IVa, hot and humid, at 30°C and 65 per cent relative humidity, and Zone IVb, hot and very humid, at 30°C and 75 per cent. WHO Prequalification assumes that prequalified medicines will be used across the sub-zones of Zones III and IV, and asks for long-term data at Zone IVb conditions unless an applicant justifies otherwise.

The critical word is long-term. A 30°C testing condition represents the average a product is expected to live through, not a ceiling it will never cross. Pharmacists handle that gap with mean kinetic temperature, a weighted average that gives more weight to hot periods because degradation speeds up with heat. A short spike above 30°C may do little. A hot season that pushes the average up for weeks is a different matter.

What temperatures do medicines actually sit in after the cold chain?

Hotter than the label, often, and for longer than a spike. The best measured evidence comes from vehicles, because emergency services have studied their own drug boxes. A prospective multicentre study of medication storage on US ambulances logged temperatures above 30°C in every drug storage box during the summer months, for between 3 and 29 per cent of the time depending on the site. Mean kinetic temperatures by location for the whole period ranged from 21°C to 30°C. A literature review of out-of-hospital storage traces the concern back to the mid-1980s.

Those are vehicles in a temperate country, with engineering and scrutiny behind them. For the storerooms and homes where medicines spend most of their time in hot countries, we could not find comparable measured data. That is a gap, and it is the one that matters most: a patient on a three-month supply keeps it at home, through a season that may run above 30°C indoors for weeks.

A degraded medicine is Available on paper. It is still on the shelf. It no longer does its job.

Does heat actually degrade medicines?

Some, sometimes, and it depends on the molecule. The evidence is mixed and should not be generalised. A review of the emergency medicine literature summarises it plainly: several common emergency medicines showed no significant chemical change after short exposures to extreme heat and cold. Lorazepam and succinylcholine, both normally refrigerated, showed marked degradation within as little as four weeks at ambient or vehicle temperatures.

Three limits apply to all of this evidence. It covers a small set of medicines, mostly emergency ones, not the long-term treatments most access programmes supply. Much of it tests exposures unlike real life, either very short and very hot or long at mild temperatures. And the formulation and packaging matter as much as the active ingredient, so a result for one product does not transfer to another of the same molecule. Anyone who tells you heat does or does not damage medicines in general is saying more than the evidence allows.

What can a programme do about the segment it does not watch?

Start watching it, cheaply, and reduce exposure where it is easy. None of these moves needs better science first.

• Log the storeroomPut a temperature logger in each partner storeroom and dispensing point through one hot season. The data turns an assumption into a record, and it costs less than one lost consignment.
• Shorten the home stayWhere supply chains allow, dispense smaller quantities during the hottest months so less stock sits in homes through the peak. Weigh it against the travel burden on patients, which is its own access cost.
• Say where to store itGive patients one practical instruction: the coolest place in the house, away from the roof, the stove and the window, and never in a vehicle.
• Ask for the dataAsk suppliers for Zone IVb stability data and any in-use or excursion data for the products the programme depends on. The answer tells you which products to worry about first.

The monsoon half of the same problem, where flood days break the chain before the heat has a chance to, is set out in the monsoon cold chain.

What should change first?

Stability testing was built on climate averages, and the averages are moving. The testing standards will follow in time. Programmes do not need to wait for them. The segment between the last cold point and the patient is the one nobody measures, and it is the cheapest one to start measuring: a logger in a storeroom, a question to a supplier, a sentence to a patient. That record is what will show, for the first time, whether the medicines a programme delivers are still doing their job when they are taken. The broader argument is in climate change is a healthcare story, and the heat terms are defined in the glossary.

Questions worth asking after this

What is Zone IVb in stability testing?

Zone IVb is the hot and very humid climatic zone defined by the WHO Expert Committee in 2005. Long-term stability testing for Zone IVb markets runs at 30 degrees Celsius and 75 per cent relative humidity, and WHO Prequalification asks for Zone IVb data by default for prequalified medicines.

Does heat degrade medicines?

It depends on the molecule, the formulation and the exposure. Studies of emergency medicines found some showed no significant change after short extreme exposures. Others, including lorazepam and succinylcholine, degraded within weeks at ambient or vehicle temperatures. The evidence does not support general conclusions across medicines.

How can a health programme protect medicines from heat after the cold chain?

By logging temperatures in partner storerooms through a hot season, dispensing smaller quantities during the hottest months where feasible, giving patients a clear storage instruction, and asking suppliers for Zone IVb and excursion stability data for key products.

Abhijith Magal

Founder, Syntropy Earth. Nine years across two global pharmaceutical multinationals in patient access and commercial roles, with health-equity work alongside the WHO-Foundation and UNICEF. He works on climate-access: where climate disruption breaks the link between patients and care. More about Abhijith →

This page describes published guidance and evidence. It is not advice on any product. Patients should follow the storage instructions on their medicine and ask a pharmacist if unsure.

Last updated: 24 September 2026