Enteral medication osmolality

Many medicines given through a feeding tube are oral liquids: syrups, elixirs, suspensions and compounded preparations. Their osmolality, the number of dissolved particles per kilogram of water, is often far higher than that of the enteral formula they are given alongside. This page collects published measured values, shows how the numbers were obtained, and walks through the mixing arithmetic.

This page does not tell you how to dilute or give any medicine. Whether, how and with what to dilute a medicine for a feeding tube is a decision for the prescriber and pharmacist. ASPEN’s enteral nutrition practice recommendations advise that “when a patient on EN requires medications, it is advisable to consult a pharmacist” [6].

Why medicine osmolality matters for tube feeding

• Medicines are a bigger osmotic load than feeds. Most standard polymeric formulas sit at a few hundred mOsm/kg (see enteral formula osmolality). Dickerson and Melnik (1988) measured 58 oral drug liquids: only 6 were at or below 1,000 mOsm/kg, and the rest ranged from 1,050 to 10,950 mOsm/kg (vapor-pressure osmometry, measured at 1:10 dilution) [4]. Shah et al. (2021) tested 52 oral medications commonly used in one US neonatal unit. 43 gave a measurable value, and 37 of those were above 500 mOsm/kg. The authors grouped their results at 500, 1,000 and 5,000 mOsm/kg for reporting; this page does not use any of these as a limit [1].
• Excipients drive much of the value. Sugars, sorbitol, glycerin, propylene glycol and alcohol add many particles. ASPEN notes that “liquid medications containing sorbitol may cause loose stools and abdominal discomfort, leading to cessation of the feeding” [6].
• Osmolality is a weak predictor of tolerance on its own. In 443 adult ICU patients, hyperosmolar medicines (> 500 mOsm/kg) given by nasogastric tube were not associated with feeding intolerance. Post-pyloric (nasoduodenal) administration was associated with diarrhoea, with a very wide confidence interval (OR 138.7, 95% CI 2.33–8,245) [7]. A number on this page is information, not a verdict on tolerance.
• Medicines are not mixed into the feed. Williams (2008): “Medications should not be added directly to the enteral formula, and feeding tubes should be properly flushed with water before and after each medication is administered” [5]. ASPEN adds that “a flushing protocol should be in place to prevent formula-drug interaction and device clogging” [6].

Infants and neonates. Most of the data below were collected in neonatal units, but this page gives no neonatal osmolality limit for medicines or feeds. For the one historical formula figure and why it is not a medicine threshold, see the infant note on enteral formula osmolality.

Measured osmolality of oral liquid medicines

All values come from Osmolar Data v1.1.3, which records each value with its original source, table and verbatim extract. Every value here is a laboratory measurement (none are labelled or calculated). “After dilution” means the value was above the instrument’s range, so the liquid was diluted, measured and multiplied back. Leong and Shaniv describe this in their methods (×2 or ×3 in Leong; ×10 in Shaniv). Shah states only that “dilutions were performed” outside its 100–1,500 mOsm/kg calibration range. For Shah and Leong, the label here is inferred from the reported value and the stated range, because neither paper marks individual rows. Those back-calculated values assume ideal dilution and are less precise.

T = table number in the source. ± values are the SD reported by Shah (triplicate). Leong and Shaniv report means only.

Methods differ. Shah used freezing-point depression (Advanced 3250; calibration range 100–1,500 mOsm/kg; triplicate) [1]. Leong used a Micro OSMETTE II (maximum 2,000 mOsm/kg; duplicate aliquots; the letter does not name the measurement principle) [2]. Shaniv used freezing-point depression on an Advanced 3320 (0–2,000 mOsm/kg; duplicates) [3]. Measured values for the same drug can differ between labs for this reason alone. See how osmometers work.

Gaps in the data. Shah reported that 9 of its 52 medications gave no result: five suspensions whose particles settled and four that “did not freeze adequately”. The paper does not say which product failed for which reason, and its Table 5 lists eight products (among them amoxicillin–clavulanate, azithromycin, phenytoin and rifampin). Clonidine 5 mcg/mL was reported only as ”< 100” mOsm/kg [1]. None of these has a numeric value, so none is shown.

Country and product variation: digoxin

The same drug at the same strength can have very different osmolality in different countries, because the product, excipients and manufacturer differ.

• US: Lanoxin elixir 50 mcg/mL (West-Ward), 5,597 mOsm/kg [1]
• Canada: Toloxin 0.05 mg/mL (Pendopharm), 3,670 mOsm/kg [2]

Same labelled strength (0.05 mg/mL), but 5,597 ÷ 3,670 = 1.53, so the US elixir measured about 50% higher. Both were measured after dilution, by different labs and instruments. Furosemide shows a similar spread: 7,757 mOsm/kg for the US 8 mg/mL Lasix solution [1] versus 9,335 mOsm/kg for an Israeli Furosemide Oral Solution USP whose strength is not given in the article text [3]. Both were measured after dilution.

The value that applies is that of the specific product in use. A figure from another country, brand or compounding recipe may not transfer.

Mixing arithmetic (water-based method)

Osmolality is per kilogram of water, so the correct mass balance uses the water each component contributes:

Osm_mix (mOsm/kg) = Σ(Osm_i × W_i) ÷ Σ W_i, where W_i is the kg of water from component i (volume × water fraction; for plain water, 1 mL ≈ 1 g).

This follows from the definition of osmolality (see osmolality vs osmolarity). Two limits apply:

• Water content of liquid medicines is rarely published. When it is unknown, the only option is the volume-weighted fallback, Σ(Osm_i × V_i) ÷ Σ V_i. That is approximate and likely an overestimate for syrups, which hold less than 1 kg of water per litre.
• Concentrated liquids do not dilute exactly linearly, because osmotic coefficients change with concentration. Every result below is an estimate; a measured value of the actual mixture is better.

The examples below are arithmetic only. The volumes are illustrative. They are not dilution recommendations, and no volume on this page is advice for any medicine or patient.

Worked example 1: unknown versus known water content

A hypothetical liquid at 4,000 mOsm/kg, 10 mL, combined in arithmetic with 30 mL of water.

• Water content unknown (fallback): (4,000 × 10 + 0 × 30) ÷ 40 = 1,000 mOsm/kg (approximate; likely an overestimate).
• Water fraction 0.70 known: water from the liquid = 10 × 0.70 = 7 g. Osmoles = 4,000 × 0.007 = 28 mOsm. Total water = 7 + 30 = 37 g = 0.037 kg. 28 ÷ 0.037 = 756.8 → 757 mOsm/kg.

The fallback overstates the result by about 32%. The difference grows as the liquid’s water fraction falls.

Worked example 2: why the product matters

Any of the arithmetic above scales with the starting osmolality. For the same calculation, the US Lanoxin value (5,597 mOsm/kg) gives a result 1.53 times that from the Canadian Toloxin value (3,670 mOsm/kg). That is the only point of the comparison: neither number says anything about how either product should be prepared or given.

Worked example 3: reading a back-calculated value

Leong measured liquids above 2,000 mOsm/kg after diluting them 1:1 or 1:2 and multiplying by 2 or 3 [2]. Shaniv multiplied a 1:10 reading by 10 [3]. A hypothetical reading of 1,000 mOsm/kg after a 1:2 dilution therefore reports as 3,000 mOsm/kg. Back-multiplication assumes ideal behaviour, so treat such values as approximate. This is one reason values above about 2,000 mOsm/kg differ between studies.

Practice questions

Q1. 5 mL of a liquid at 3,000 mOsm/kg is combined in arithmetic with 15 mL of water, and its water content is unknown. What is the fallback estimate, and which way is it biased?

Answer

(3,000 × 5) ÷ 20 = 750 mOsm/kg. This is approximate and likely an overestimate, because a syrup holds less than 1 kg of water per litre.

Q2. The same liquid has a water fraction of 0.80. Use the water-based method.

Answer

Water from the liquid = 5 × 0.80 = 4 g. Osmoles = 3,000 × 0.004 = 12 mOsm. Total water = 4 + 15 = 19 g = 0.019 kg. 12 ÷ 0.019 = 631.6 → 632 mOsm/kg. That is lower than the 750 fallback, as expected.

Q3. 240 mL of Osmolite 1.0 (288 mOsm/kg as stated by the manufacturer, water 0.849 kg/L) is combined with 60 mL of water. Compare the water-based result with the volume-weighted fallback.

Answer

• Water-based: Osmolite water = 0.240 × 0.849 = 0.20376 kg. Osmoles = 288 × 0.20376 = 58.68 mOsm. Total water = 0.20376 + 0.060 = 0.26376 kg. 58.68 ÷ 0.26376 = 222.5 → 222 mOsm/kg.
• Fallback: (288 × 240) ÷ 300 = 230 mOsm/kg, an overestimate.

See enteral formula osmolality.

Q4. Why is the US digoxin value (5,597) not the right number for a patient in Canada?

Answer

The products differ (Lanoxin elixir vs Toloxin), and so do excipients, labs and instruments. Both values were measured after dilution. The relevant figure is the one for the specific product in use (3,670 mOsm/kg for the Canadian product in Leong 2020), confirmed with a pharmacist.

Q5. A team suggests putting a liquid medicine straight into the feed bag so it “dilutes in the formula”. What does the published practice guidance say?

Answer

Williams (2008): medications “should not be added directly to the enteral formula”, and tubes should be flushed with water before and after each medication [5]. ASPEN advises consulting a pharmacist when a patient on EN needs medications [6].

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Educational content, not medical advice. The values are published laboratory measurements of specific products and may not match the product you have. The arithmetic is not a dilution, administration or tolerance recommendation. Ask the prescriber or pharmacist about any medicine given by feeding tube. See our disclaimer and how we check content.

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References

1. Shah DD, Kuzmov A, Clausen D, et al. Osmolality of commonly used oral medications in the neonatal intensive care unit. J Pediatr Pharmacol Ther. 2021;26(2):172–178. doi:10.5863/1551-6776-26.2.172. PMID 33603581. https://pmc.ncbi.nlm.nih.gov/articles/PMC7887878/ (Tables 1–5; values via Osmolar Data v1.1.3, spot-checked against the PMC tables 2026-10-18)
2. Leong A, Gordon A, Alshaikh B, Yusuf K, Dersch-Mills D. Osmolality of medications administered in the neonatal intensive care unit. Can J Hosp Pharm. 2020;73(4):288–289. PMID 33100361. doi:10.4212/cjhp.v73i4.3033. https://pmc.ncbi.nlm.nih.gov/articles/PMC7556401/ (Table 1) (accessed 2026-10-18)
3. Shaniv D, Teitlboim S, Kramer-Haimovich H, Goldwaser Y, Matok I, Perlman A, Hershkovich-Shporen C. Osmolality of oral liquid medications intended for neonates and infants: a laboratory-based study. Drugs Ther Perspect. 2025;41(11–12):555–567. doi:10.1007/s40267-025-01205-w. https://link.springer.com/article/10.1007/s40267-025-01205-w (values quoted in Results/Discussion text) (accessed 2026-10-18)
4. Dickerson RN, Melnik G. Osmolality of oral drug solutions and suspensions. Am J Hosp Pharm. 1988;45(4):832–834. doi:10.1093/ajhp/45.4.832. PMID 3376967. https://pubmed.ncbi.nlm.nih.gov/3376967/ (accessed 2026-10-18)
5. Williams NT. Medication administration through enteral feeding tubes. Am J Health Syst Pharm. 2008;65(24):2347–2357. doi:10.2146/ajhp080155. PMID 19052281. https://pubmed.ncbi.nlm.nih.gov/19052281/ (accessed 2026-10-18)
6. Boullata JI, Carrera AL, Harvey L, et al. ASPEN Safe Practices for Enteral Nutrition Therapy. JPEN J Parenter Enteral Nutr. 2017;41(1):15–103. doi:10.1177/0148607116673053. PMID 27815525. https://pubmed.ncbi.nlm.nih.gov/27815525/ (accessed 2026-10-18)
7. Wesselink E, Koekkoek KWAC, Looijen M, van Blokland DA, Witkamp RF, van Zanten ARH. Associations of hyperosmolar medications administered via nasogastric or nasoduodenal tubes and feeding adequacy, food intolerance and gastrointestinal complications amongst critically ill patients: a retrospective study. Clin Nutr ESPEN. 2018;25:78–86. doi:10.1016/j.clnesp.2018.04.001. PMID 29779822. https://pubmed.ncbi.nlm.nih.gov/29779822/ (accessed 2026-10-18)