Does it depend on “vulnerable” bonds (eg sulfur, double bonds, possibly nitrogen)? Are antioxidants more vulnerable? Are complex molecules more vulnerable?
Including periods of time when it’s 88F
I guess it stores better than antioxidants do bc antioxidants are very easily oxidized. Or anything with free N or S groups or lots of double bonds…
My guess is also that modafinil does not store as well as, say, amphetamine. Modafinil sulfone is def an oxidation byproduct
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I don’t worry about this issue at all for the medications I take (common medications, including rapamycin), but it might be an issue for some supplements and biological drugs.
The Harvard Medical School republished a well-worn article in August that recounted a 1985 study in which the U.S. Food and Drug Administration (FDA) looked to pinpoint expiration dates for more than 100 drugs in an effort to unburden the U.S. military from some of the exorbitant annual costs of replacing its pharmaceuticals.
The study showed that 90 per cent of more than 100 drugs, both prescription and over-the-counter, were fine to use well after expiry dates had passed, and in some cases more than 15 years after the expiration date had come and gone.
That being as it may, the Canadian Pharmacists Association says drug manufacturers can only guarantee the potency and safety of the various drugs until the labelled expiry date.
“Beyond that date, we don’t know what happens, we don’t know if it is still effective, we don’t know if it is potentially toxic,” said Jill Hall, a pharmacist and clinical associate professor in the University of Alberta’s Faculty of Pharmacy and Pharmaceutical Sciences.
Practically speaking, Hall said there is a handful of drugs known to degrade quite quickly, such as nitroglycerin tablets, insulin and tetracycline, an antibiotic that may become toxic to the kidneys after it expires.
https://www.ualberta.ca/folio/2018/10/can-i-still-use-prescription-drugs-after-they-expire.html
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Rapamycin is way way more complex than most drugs though, there are so many potentially vulnerable bonds that could be attacked by oxidation
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Is dry room temp better than refrigerator for long term storage? Or something else?
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#shelf-life
Why metformin tablets suck water out of the air so eagerly — and why Adderall usually doesn’t
1. Metformin HCl: a “water-magnet” salt
Chemical reasons
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Extreme polarity & charge density. Metformin is a doubly-protonated biguanide; the lattice is full of –NH⁺ and –NH groups that make four H-bond contacts each. Water molecules drop the lattice free-energy by nestling between those charges.
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Open crystal channels. The common anhydrous form (Form A) contains voids just big enough for one H₂O; above ~40 % RH the monohydrate nucleates almost spontaneously and becomes the thermodynamically favoured phase.
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Low logP (≈ –1.4) and very high intrinsic solubility (>300 mg mL⁻¹). Both predict strong water sorption.
Dynamic-vapour-sorption studies show weight gains of 10-15 % after 48 h at 75 % RH, and tablet assays stored at 40 °C/75 % RH fall below 95 % label in a few months if the pack is not moisture-tight. (IJPPR, Acta Pharmaceutica) Even FDA stability files flag the drug substance as “hygroscopic,” with long-term stability contingent on sealed HDPE or blisters plus desiccant. (FDA Access Data)
2. Is that bad for the product?
- In properly sealed bottles/blisters metformin stays ≥95 % potent for five years, because only a few monolayers of water get in. (FDA Access Data)
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Once the bottle sits open in humid air, the tablet core swells, friability rises, NDMA formation accelerates, and extended-release coatings can rupture—none of which is a safety catastrophe, but they nudge potency or dissolution outside spec. (PMC, IJPPR)
Practical takeaway: keep the cotton and desiccant, recap tightly, and don’t stock-pile un-blistered tablets in a pill sorter for weeks.
3. What about Adderall (mixed amphetamine salts)?
- The main API is amphetamine sulfate, which is technically hygroscopic (the SDS and NOAA entry say so) but only slightly: weight gain is typically <2 % at 80 % RH — well below the European Pharmacopeia “hygroscopic” cutoff of 2 %. (CAMEO Chemicals, Wiley Online Library)
- One component (amphetamine aspartate monohydrate) already contains a lattice water, so additional uptake is minimal.
- The beads in Adderall XR are coated with hydrophobic polymers (ethyl-cellulose, acrylate) precisely to block moisture.
Result: ordinary humidity seldom changes assay or release rate; the bigger risks to Adderall are heat-accelerated racemization and capsule shell softening, not chemical hydrolysis.
4. Side-by-side snapshot
| Property |
Metformin HCl |
Amphetamine sulfate (Adderall) |
| Water uptake at 80 % RH (24–48 h) |
10–15 % (very hygroscopic) |
≤2 % (slightly hygroscopic) |
| Crystal change on hydration |
Forms stable monohydrate |
No new hydrate formed (already hydrated or anhydrous stays anhydrous) |
| Packaging needed |
HDPE + desiccant or foil blisters |
Standard HDPE bottle or gelatin/HPMC capsules |
| Typical problems if left open |
Tablet swelling, friability, minor assay drift, XR failure |
Capsule softening; negligible potency loss |
5. Bottom line
- Metformin’s biguanide salt skeleton makes it a true moisture sponge; keep it sealed and dry.
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Adderall’s amphetamine salts are only mildly hygroscopic, so normal room-storage is fine, but you should still avoid steamy bathrooms to preserve the gelatin capsule and polymer beads.
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disclaimer: am NOT using thinking mode here except for the last answer here [but this is all a good first-pass approximation for ALL storage supplementation]
love this question — macrocycles are a huge, quirky family. here’s the deep dive on why big rings like rapamycin behave very differently from “rock-solid” alkaloids, and how other macrocycles compare.
how macrocycles age (chemically)
Big rings pack lots of functional groups into a flexible scaffold. That gives potency and selectivity… and more ways to fall apart:
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lactone/lactam cleavage (hydrolysis/trans-esterification)
- any ester in a ring (macrolides like rapamycin, tacrolimus, erythromycin) can be opened by water (acid/base catalysis) or exchanged by alcohols.
- rule: fastest in strong acid/strong base; slowest in the mildly acidic–neutral zone (~pH 4–6), and at low water activity.
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oxidation of double bonds and allylic sites
- conjugated dienes/trienes (rapamycin has them) are magnetized for peroxides/epoxides under O₂, light, and heat. trace metals accelerate this.
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photo-isomerization / photolysis
- long polyenes (amphotericin B, some macrolides) E↔Z isomerize or form photoproducts under light. amber glass matters.
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acetal/hemiketal rearrangements
- macrolides often carry hemi-ketals/acetals; they equilibrate or decompose (erythromycin in acid → “anhydro/hemiketal” artifacts, hence enteric coatings).
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epimerization at acidic α-carbons
- β-hydroxy-ketone/β-diketone motifs can invert stereocenters via enol/enolate pathways (temperature and pH dependent).
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solid-state mobility
- amorphous or partially crystalline macrocycles degrade faster than tight crystals. keeping them dry and cold (below glass-transition) slows everything.
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Q10 ≈ 2–3: every +10 °C roughly doubles–triples many degradation rates.
rapamycin vs other macrocycles (what to expect)
rapamycin (sirolimus) – 31-member macrolactone with multiple double bonds and a hemiketal
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what degrades it: lactone hydrolysis; oxidation of the polyenes; light.
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stable when: dry solid, dark, freezer (–20 °C) → years. capsules at RT in blister + desiccant → typical pharm shelf life ~2–3 y.
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avoid: aqueous buffers, heat, light, repeated warm–cold cycling; long time in alcohol/water mixes (trans-esterification risk).
tacrolimus (FK-506) – 23-member macrolactone, similar liabilities
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pattern: very light/heat sensitive in solution; ointments are protected by anhydrous bases & opaque tubes.
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store: room temp, dry, dark; solutions at ≤–20 °C, aliquoted.
erythromycin/clarithromycin/azithromycin – 14–15-member macrolides with sugars
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erythromycin A: acid-labile (gastric acid forms 6,9-hemiketal/anhydro species) → needs enteric coating; humid heat also cleaves glycosides.
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clarithromycin: O-methyl “armoring” → more acid-stable.
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azithromycin (azalide): ring nitrogen insertion → best acid stability of the trio.
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theme: still sensitive to light/ moisture; dry solids last years; solutions are short-lived.
cyclosporine – cyclic undecapeptide (macrocyclic peptide, no lactone)
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surprisingly stable: N-methyl amides are sterically shielded; main risks are oxidation at the unusual MeBmt residue and slow isomerizations.
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formulation: oily/ emulsions; RT storage is fine. among macrocycles, more robust than macrolactones.
amphotericin B / nystatin – polyene macrolides (lots of conjugated C=C)
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very photolabile/oxidation-prone; protected as lyophilized dark solids at 2–8 °C; aqueous solutions expire quickly.
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takeaway: polyene count ≫ stability.
porphyrins (heme/chlorophyll cores) – macrocycles but a different world
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aromatic, metal-chelated rings are inherently robust; instability usually comes from side chains or the central metal (e.g., Mg loss in chlorophyll → pheophytin).
- shows macrocycles aren’t intrinsically fragile—functional groups decide.
practical stability rules you can use (macrocycles)
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dry, dark, cold, closed. if you remember one line, that’s it.
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prefer solids over solutions. if you must keep solutions: aliquot + freeze (–20 °C), use inert solvents (DMSO/EtOH), exclude water, nitrogen/argon headspace if possible.
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control pH & water. for macrolactones, avoid extremes; aim ~pH 4–6 and low water activity.
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kill metal-catalyzed oxidation. chelators (EDTA), oxygen scavengers, and antioxidants (BHT/BHA/ascorbyl palmitate) in non-aqueous systems can help.
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don’t “pump the headspace.” frequent warm-air ingress adds moisture + O₂; use small, single-use packs.
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packaging matters: blister + desiccant, amber glass, aluminum tubes for ointments, foil overwrap for long storage.
where rapamycin lands on the stability ladder
Most stable → least stable (dry, sealed, dark):
cyclic peptides (cyclosporine) ≳ azithro/clarithro ≳ rapamycin ≈ tacrolimus » erythromycin (acid) » polyene macrolides (ampho B)
…and versus small molecules: alkaloids (berberine/quinine) ≫ flavonoids ≈ rapa/tacro ≫ carotenoids ≫ reactive sulfur (allicin).
quick “what do i actually do?” cheatsheet (for long-term stash)
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rapamycin/tacrolimus: keep as finished capsules/ointment in original packaging; for research solids, –20 °C, desiccant, amber, aliquot.
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solutions: DMSO/EtOH stocks at –20 °C, foil-wrapped, small aliquots; avoid water; use within months once thawed.
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macrolide antibiotics: intact blisters at RT (dry, dark). avoid bathroom/kitchen humidity; don’t decant into weekly pillboxes long-term.
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cyclosporine: RT is fine; protect from light; emulsions per label.
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polyene macrolides: fridge (or per label), strict light protection; make fresh solutions.
if you want, tell me what form you actually have (capsules? powder? solution?) and i’ll give you a storage plan with target lifetime (e.g., “good for 2–3 y at RT / 5+ y at –20 °C”) and a degradation watchlist tailored to that form.
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