Sublingual Therapeutics: A Gateway To Enhanced Drug Absorption

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Jul 11
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Updated: Aug 11

Technical Review Article | Open Access | Published 11th July 2025

Sublingual Therapeutics: A Gateway To Enhanced Drug Absorption

Neeraj Vishwakarma, Pravin Kumar Sharma*, Sumeet Dwivedi, Ashish Gupta and G.N. Darwhekar

Acropolis Institute of Pharmaceutical Education and Research | EJPPS | 302 (2025) https://doi.org/10.37521/ejpps302012

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Abstract

Sublingual drug delivery holds promise for facilitating quicker and more direct absorption of medications into the bloodstream. The sublingual area in the mouth enhances drug absorption, bypassing the liver's first-pass metabolism, resulting in higher bioavailability and improved patient compliance. Sublingual medications can help children, the elderly, and individuals with mental health disorders or those that have dysphasia. Over time, these new options help in improving treatment management by making medication easier and more convenient. Because of their potential for emergency treatment, there is a growing demand for sublingual drug delivery. In contrast to the oral cavity's palatal and buccal areas, the sublingual region is more permeable; thus, fast-dissolving sublingual tablets have the potential to significantly outperform existing treatment options for certain patient populations, including elderly and paediatric patients.

Keywords: Sublingual drug delivery, absorption of drug, superior permeability, avoid hepatic first-pass metabolism, bioavailability.

Introduction

In sublingual drug delivery medication is placed under the tongue and administered drugs pass through the tongue and mouth surface into the bloodstream. Sublingual administration offers faster action and enhanced compliance of patients due to its higher permeability in the sublingual area of the mouth, making it an effective method for delivering immediate pharmacological effects (German, R.Z. et al., 2006).

Patients with dysphagia, especially those who have trouble swallowing, can benefit from immediate pharmacological effects through sublingual drug delivery. The medication is positioned beneath the tongue as shown in figure 1 and enters the bloodstream via the oral cavity floor and the tongue's ventral side. Only a hypodermic injection can match its effectiveness, which is three to ten times more than the oral route (Balogh, M.B. et al., 2011).

Figure 1: Sublingual administration (Goswami, T.et al., 2018) — **Figure 1**: Sublingual administration (Goswami, T.et al., 2018)

The buccal region has a higher permeability than the palatal region, while the sublingual oral cavity has a higher permeability than both. An immediate pharmacological effect is provided by systemic drug delivery via the sublingual route. These sublingual tablets are compressed, flat, and tiny, and they are designed to be dissolved quickly in the saliva; fast-dissolving, bioadhesive, and lipid matrix tablets are common varieties of sublingual drug delivery (Prathusha, P. et al., 2017).

Sublingual administration of drugs like opioid analgesics, steroids, and cardiovascular medications offers benefits over oral administration due to less salivary enzyme breakdown and quicker, simpler delivery. Oral routes can be affected by bile, enzymes, and stomach acid, making sublingual administration suitable for certain substances (Sharma, P.K. et al., 2017).

THE ORAL CAVITY

The mouth, also known as the buccal or oral cavity, is the opening end of the human alimentary canal, providing the entrance to body nutrients and food consumption. It is crucial for swallowing, chewing, and entering the gastrointestinal system. The oral cavity extends from the lips to the oropharynx, supporting the tongue and mouth floor through the mylohyoid muscle. The oral vestibule is a small slit-like area between the lips and teeth, while the larger part is the oral cavity proper. The pharynx is a part of the respiratory and digestive system that changes shape during breathing, swallowing, and speaking. The oral cavity includes the tongue, dentition, mucosal and salivary glands (Sain, M.V.S. et al., 2023).

The oral route's unique environment and potential for drug delivery have made it widely accepted (Nibha, K.P. et al., 2012).

THE SUBLINGUAL GLAND

Sublingual glands, located beneath the tongue, produce various secretions, which lubricate the mouth for chewing and swallowing food. They also help prevent food lodgement in the throat. Salivary glands, which are complex, tubuloacinar, exocrine, and merocrine, provide primarily salivation. Major and minor salivary glands produce saliva, a mixture of water, inorganic, and organic materials (Patel, J.K.et al., 2022, Del Pinto et al., 2008).

The fluid volume, pH, and composition of the oral cavity are all influenced by saliva secretion, which also shapes its physiological environment. The parotid, submaxillary, and sublingual glands are the three main salivary glands that have been shown to promote saliva secretion. Location of sublingual gland is shown in figure 2.

Figure 2: Position of sublingual gland (Sumi, M.et al., 1999) — **Figure 2**: Position of sublingual gland (Sumi, M.et al., 1999)

Salivary enzymes and secretions control oral microbial flora through pH maintenance and enzyme activity. Parotid and submaxillary glands produce watery secretions, while sublingual glands produce viscous saliva and enzymatic activity. Saliva aids swallowing and prevents tooth reactions. It has a small volume of 1.1 millilitres, resulting in lower drug release than the gastrointestinal tract. Its flow rate is influenced by time of day, stimulus type, and intensity. Sublingual glands produce mucin for chewing and swallowing food, allowing chewable food to pass into the throat and digestive tract (Thulluru, A. et al., 2019, Naimish, A.S. et al., 2013).

DRUG DELIVERY THROUGH THE SUBLINGUAL ROUTE

Drugs penetrate into the bloodstream through the tissues of the sublingual region, which is also known as the pharmacological route of administration. This route brings drugs into direct contact with oral mucosa, entering the bloodstream and enhancing the permeability and bioavailability of the formulation. Children and the elderly experience dysphasia or difficulty swallowing and it is a promising strategy for negotiating these issues. This area can easily surpass hypodermic injection and has a 5-10 times higher drug penetration rate than other delivery methods. The ideal physicochemical properties of drug are described in table 1. Some medications have a high first-pass metabolism, which makes their oral dosage forms less bioavailable; these medications are better suited for sublingual administration.Sublingual administration is also used to administer medications that are not administered in parenteral preparations (Kanade, T. et al., 2023).

Table 1: Ideal physicochemical properties of drugs for sublingual delivery (Thulluru, A. et al., 2019).

Physiochemical properties	Allowed range
Dose	Not more than 20 mg
Molecular mass	Moderate-small (163.3 to 342.3 g/mol)
Pka	< 10 for basic drugs;>2 for acidic drugs
Log P	1.6 to 3.3
Stability	Excellent stability in water and saliva
Taste	Not overly bitter
Lipophilicity	Lipophilic

MECHANISM OF SUBLINGUAL ABSORPTION

Figure 3 shows the mechanism of absorption of sublingual tablets. When the sublingual tablet is placed under the tongue, it is absorbed through mucosal epithelial cells. These cells absorb the drug and it then reaches the connective tissue. The drug then crosses the connective tissue and goes directly into the veins (nostrils) from where it gets mixed in the bloodstream. In this way the drug bypasses the first-pass metabolism of the liver, due to which its effect is quicker and enhanced (Sain, M.V.S. et al., 2023, Narang, N. et al., 2011).

Figure 3: Permeation of sublingual tablet (Singh, M. et al., 2012) — **Figure 3**: Permeation of sublingual tablet (Singh, M. *et al.,* 2012)

In general, the oral mucosal membrane is a type of leaking epithelium that lies between the intestinal and epidermal. The sublingual mucosa being relatively thin and nonkeratinized, as the degree of keratinization of these tissues increases, the permeabilities of the oral mucosa decrease. A medication needs to have a slightly higher lipid solubility in order to be completely absorbed via the sublingual pathway (Hooda, R. et al.,2012).

ELEMENTS THAT IMPACT SUBLINGUAL ABSORPTION

Solubility in secreted saliva

The solubility of saliva plays an important role in sublingual absorption. If the drug dissolves well in saliva, it is easily absorbed by the mucosal membrane. Poor solubility can slow or limit absorption. For the drug to be absorbed, it must have both high lipid solubility and biphasic solubility, meaning it must dissolve in aqueous buccal fluids.

Attaching to the oral mucosa

Systemic availability is low for medications that adhere to the oral mucosa.

Saliva's pH and pKa value of drug

If the range of pKa for an acid is more than two and for a base less than ten, the drugs will be absorbed by oral mucosa at pH 6.8.

Drug lipophilicity

A drug needs to have a slightly greater solubility in lipids than that necessary for GIT absorption for it to be completely absorbed through the sublingual pathway

The thickness of oral epithelium

The buccal epithelium is thicker than the sublingual epithelium and thus drug absorption happens faster from this route (Sah, S.et al., 2016, Rode, P.A. et al., 2022).

BENEFITS OF SUBLINGUAL MEDICATION

· Bypassing the liver, the pH and digestive enzymes of the middle gastrointestinal tract that protect the drug from degradation

· Avoid hepatic first-pass metabolism and thus provide high efficacy at low dose

· Sublingual dosage forms are frequently useful in emergencies, such as heart disorders and asthma, due to their quick action.

· The formulation can be discarded if therapy needs to be stopped, and a comparatively quick and fast action is obtained in comparison to the oral route.

· Another benefit they offer is quick dissolution or disintegration without the need for water or chewing in the mouth (Fu, Y., Yang et.al 2004).

DISADVANTAGES OF SUBLINGUAL MEDICATION

· Patient acceptability in terms of taste, irritability, and "mouth feel" is a concern for both the local and systemic action.

· Highly ionized drug administration is not allowed.

· If consumed, that quantity must be processed through first-pass metabolism and handled similarly to an oral dosage.

· A high dose of drug cannot be given due to limited surface area (Patel, P.M. et al., 2011, Bhati, R. et al, 2012).

METHODS OF PREPARATION OF SUBLINGUAL TABLETS

Techniques for making sublingual tablets consist of direct compression, sublimation, freeze-drying, or spray drying. Direct compression is the best technique because it is less expensive, requires less labour, and uses less power. In addition, it improves tablet stability, gets rid of high compaction pressure, and gets rid of granulation, pretreatment, and variations in the wet granulation process. Other techniques, such as spray and freeze-drying, are also feasible (Sheeba, F.R. et al., 2009).

Direct compression method

This method is commonly used to prepare sublingual dosage forms because it is simple and economical. Direct compression is the most effective technique for drugs that are heat labile. In this, we use a dry binder, sweeteners, flavours, lubricant, a super disintegrant (such as crospovidone, microcrystalline cellulose, etc.), and directly compressible and soluble ingredients (Prathusha, P. et al., 2017).

Direct compression offers good mechanical strength and rapid disintegration, making it an economical and effective method of producing sublingual tablets. It requires proper flow characteristics and cohesive strain, and it works well with heat-labile medications. Nowadays, a lot of bulking agents are made of sugar because of its high solubility, sweetness, and pleasing mouth feel (Rai, T.P. et al., 2019).

The main technical problems with the directly compressed method are as follows:

· The tablets' high variance in weight and dosage

· The weak mechanical properties of the tablets

· Their lamination and capping

· Powder material's sticking or adhesion to the tips of the punches

· High friction when ejecting tablets

The design, state, and properties of the tablet press and powder are crucial for successful tabletting operations for direct compression. The powder must have sufficient physical characteristics, be well-maintained, and have flowability, compressibility, compactability, homogeneity, and segregation (Singh, M. et al., 2012).

Moulding method

In this method tablets are moulded using hydro alcoholic solvent and water-soluble ingredients under pressure that is lower than that of traditional tablet compression. The solvent is eliminated through air drying. The tablet's mechanical robustness is progressively increased through binding agents like poly vinyl pyrrolidone, sucrose, or acacia. Tablets can be rapidly dissolved and disintegrated using wet granulation, which increases the tablets' solubility. But it can result in issues with flavour masking and mechanical robustness (Yarmal, R.V. et al., 2022).

The process of lyophilization

Lyophilization is a dehydration process that removes water from pharmaceutical products under a vacuum. Also referred to as freeze-drying, it is used for creating rapidly disintegrating tablets that are beneficial for patients who do not swallow properly. Benefits of this process are as follows:

· It increases pharmaceutical products' stability

· Because drying occurs at a very low temperature, enzyme activity is suppressed

· The volume of the final dry product is equal to that of the initial solution

· Produces highly soluble products (Singh, M. et al., 2012).

Hot melt extrusion

Hot melt extrusion is a type of pharmaceutical manufacturing technique that uses heat and pressure for the melting and mixing of APIs. When materials are mixed uniformly and consistently in a twin-screw extruder, the dissolution rate and bioavailability of medication formulations that are poorly soluble in water can be increased. This improves mixing to create a uniform solid with the medication scattered finely. Figure 4 shows the hot-melt extrusion (HME) process, which is used in the pharmaceutical industry to make drug-loaded films or tablets (Thulluru, A. et al., 2019).

EVALUATION CRITERIA OF SUBLINGUAL TABLETS

Together with a few specialized tests, the evaluation parameters of the sublingual tablets listed in the pharmacopoeias must be performed. Once a tablet is prepared, its quality is determined by using the evaluation studies and parameters described below:

Overall appearance

For the tablet to be accepted by the patient, its "elegance" and visual identity, which include its physical defects, taste, texture, size, shape, colour, odour, consistency, identifying marking are essential organoleptic characteristics that are used in its evaluation (Kumar, D. et al., 2018).

Size or shape

It is possible to monitor and regulate the tablet's dimensions. Size and shape are measured by a micrometer, sliding caliper scale, etc. Tablet thickness should be controlled within a standard range (Saheb, R. et al., 2017).

Hardness

The hardness of the tablet is determined by the force applied across its diameter within the breaking range. How resistant a tablet is to chipping, snicking, or breaking when it is stored, altered, and handled before use depends on its hardness. The tablet's hardness is measured using the Monsanto Hardness Tester (Dhangar, R. et al., 2017).

Weight uniformity

The I.P. procedure is used to ensure weight uniformity; 20 tablets are taken, and a digital weighing balance is used to measure each tablet's weight separately and collectively. A single tablet's average weight is calculated from the total weight (Grover, I. et al., 2012).

Thickness

Using a digital vernier caliper, the thickness of randomly chosen tablets from each formulation is measured in millimetres and the mean value is calculated (Sarkhejiya et al., 2013, Varshosaz, J. et al., 2015).

Time of In-vitro dispersion

By dropping a tablet into a beaker filled with buffer, the in-vitro dispersion time is determined. Three tablets are chosen at random from each formulation, and an in-vitro dispersion time is measured as per the standard range of the pharmacopeia (Prathusha, P. et al., 2017).

In-vitro disintegration time

For estimating the disintegration time of tablets for sublingual use, 6 tablets are placed in disintegration tubes separately and distilled water used as a disintegration medium at 37ºC ± 2ºC. The amount of time in seconds that the tablets disintegrate completely without leaving any palpable mass in the apparatus is noted (Aghera, N.J. et al., 2012, Sharma, P.K. et al., 2020).

In-vivo studies

These are performed for analysis of pharmacokinetic data and assessment of bioavailability. Rabbits are among the few lab animals without keratinized mucosa, making their mucosal tissue beneath the tongue very comparable to that of humans and compatible for in-vivo evaluations. For in-vivo studies, plasma data can be used to directly determine the time to Tmax, Cmax and AUC which is followed by the bioavailability (Nayak, B.S. et al., 2017).

Measurement of pH

The pH of the tablet is measured by placing the electrode of a pH meter against the tablet's surface and letting it equilibrate for one minute. The pH is determined by letting the tablet remain in contact with one millilitre of water for two hours at room temperature (Prathusha, P. et al., 2017).

Content uniformity

The assay method for the active ingredient is used to determine the content uniformity of tablets chosen at random from every batch. The tablet is dissolved in a solvent and made into a solution, then analysed spectrophotometrically and the drug content is calculated (Parmar, R. et al., 2022).

Friability

The Roche friabilator is a tool used to quantify friability. It is used by inserting a tablet that has been previously weighed into the friabilator. The friabilator consists of a rotating plastic container that drops the tablets six inches with each rotation at 25 rpm. Rotation of the tablet within the friabilator is continued for a minimum of four minutes. By reweighing a test tablet after dusting, it calculates the weight loss and displays the results in percentages that indicate friability (Prajapati, S.T. et al., 2012).

% Friability = Tablet weight loss/Initial weight of tablet ×100

Conclusion

Sublingual drug delivery is a quicker and more effective method for delivering medications, making it suitable for young children, the elderly, and patients with swallowing issues. Compared to oral ingestion, a greater percentage of each dose is absorbed by these products, which also reach peak blood levels more rapidly. A sublingually administered drug reaches peak blood levels within 10-15 minutes with a high percentage of absorption. Some of the sublingual formulations are tablets, lozenges, films, sprays, and drops. Comparing sublingual tablets to traditional oral tablets, the former enhances drug bioavailability, decreases the time for therapeutic response, and improves patient compliance.

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