Buyer's Guides
A cap is the cheapest part of the container and the one most likely to cause a problem. Neck size, liner and closure type are three separate decisions.
OMS Clinical Team
Product Specialists
Three things decide whether a closure is right: the neck size it fits, whether it needs to be child-resistant, and what the liner is made of. Getting any one wrong causes a different problem, and they are easy to confuse when ordering.
Neck finishes are quoted as a diameter in millimetres, commonly 28mm, 38mm and 58mm, or as a GL number such as GL18 for laboratory-style threads. The number describes the outside diameter of the bottle neck, not the cap. Two bottles of identical capacity can have different necks, so the size of the bottle tells you nothing about which cap fits.
If you buy bottles and closures from different suppliers, confirm the neck finish on both. It is the single most common cause of a delivery that cannot be used.
Child-resistant closures are designed to be difficult for young children to open while remaining usable by adults. In UK practice they are expected for most solid dose medicines dispensed to patients, with exceptions where the patient cannot manage them. Where a patient has arthritis or reduced grip, a non-child-resistant cap is often the appropriate choice, and that decision should be recorded.
The usual mechanism is push-and-turn or squeeze-and-turn. Both need a positive action that a small child is unlikely to combine correctly. They also need the cap to be properly seated, which is why a rushed hand can leave a closure that looks fastened and is not.
A tamper-evident closure shows whether the container has been opened since it left you. Most use a ring that separates from the cap on first opening, so the evidence is permanent and obvious. Like a tamper-evident bag seal, it does not prevent opening. It records it.
These matter most where the container travels: deliveries, care home supply, transfers between sites. For an item handed across the counter they add little.
The liner is the disc inside the cap that forms the seal. PVDC-lined closures use a coating chosen for its barrier properties, resisting moisture and gas transfer better than an unlined cap or a plain wad. That matters for hygroscopic tablets, for anything where oxidation is the failure route, and for longer storage.
Count how many neck finishes you genuinely use. Most dispensaries can standardise on two or three, which simplifies ordering and stops caps being fitted to the wrong bottles under pressure. If you find yourself carrying six, look at whether the bottle range can be rationalised first.
Order closures in the same rhythm as the bottles they fit. Running out of 38mm caps while holding a thousand 38mm bottles is a common and entirely avoidable stock-out.
The common failure is not a faulty cap. It is a cap that was cross-threaded or under-tightened during a busy dispensing run. A cross-threaded closure feels fastened, sits slightly askew, and leaks in a bag. It is worth training staff to start the thread by turning the cap backwards until it drops into the groove, then forwards. It takes a second and removes most cross-threading.
Under-tightening is harder to see. With child-resistant push-and-turn caps in particular, a closure can engage the outer shell without the inner one seating, which leaves it neither sealed nor child-resistant. A quick downward press and a confirming turn catches it.
A child-resistant closure that a patient cannot open leads to one of two outcomes, both bad: they do not take the medicine, or they decant it into something else at home. Neither is safer than supplying an easier closure in the first place. Where a patient tells you they struggle, believe them and record the change.
The same applies in care homes, where staff open many containers a day. Ask what they are actually using rather than assuming the default is working.
