PTCB Guide to Liquid Dosage Forms.

Liquid Dosage Forms as a Topic of the PTCB Exam.

For the PTCB exam, pharmacy technicians are required to understand the various different dosage forms. We already talked about solid dosage forms, such as tablets and capsules. Today, we learn more about the key liquid dosage forms that get tested.

We have all at some point administered a liquid dosage form. Whether it is a cough medicine or gargling a solution to relieve symptoms affecting the back of the throat – liquid dosage forms are widespread. As a pharmacy technician, you will be handling these medicines with a liquid dosage form on a daily basis.

Among the many different liquid dosage forms, the most common include:

• Suspensions
• Solutions
• Emulsions
• Elixirs
• Tinctures
• Sprays

Advantages / Disadvantages of Liquid Dosage Forms.

Advantages of liquid dosage forms include:

1. Liquid dosage forms are effective at producing a more rapid response. Tablets, in contrast, need to be broken down by the body before being absorbed.
2. Liquid dosage forms are easier to swallow. This is particularly advantageous for younger and older patients.
3. Liquid dosage forms make it easier to dose. In other words, doses can be changes easily given the liquid formulation – a 5mL or 10mL or 15mL dose can be easily changed. The same cannot be said of solid dosage forms which are fixed and more difficult to adjust.
4. Liquid dosage forms are effective in a wider range of circumstances than solid dosage forms. For instance, liquids can be applied direct to the eye and ear, whereas solid dosage forms cannot.

Like any dosage form, though, liquid formulations are not without their disadvantages.

Disadvantages include:

1. Some liquid medicines have unpleasant odors and smells. These are often insufficiently masked by additive ingredients.
2. Liquid doses require preservatives because liquids readily produce the conditions for microorganisms to thrive.
3. Added prescription instructions are often needed. For example – suspensions need to be shaken before use, otherwise, all sediment medicine remains at the bottom of the bottle. Furthermore, refrigeration may be required.
4. Risk of dose errors. Patients may use the incorrect spoon and deliver an inaccurate dose. This may happen regularly unless patients are first correctly counseled on how to deliver the correct dose.
5. Liquid dosage forms are more inconvenient for patients because they need to carry much bulkier medicines and medicines that require the use of a spoon.
6. Loss of potency. Liquid dosage forms lose potency and deteriorate at a far faster rate than other dosage forms, such as tablets.

Liquid formulations are structured in the same way as most other medicines – namely, that there is a small amount of active ingredient (drug) dispersed throughout a larger medium. In the case of liquid formulations, active ingredient is dispersed throughout a vehicle.

Note – one of the most common kinds of PTCB exam question is to ask about excipients. This includes vehicles. It is important that students understand the definitions and differences between each kind of excipient!

This “vehicle” could be sugar-based – for example: this is the case with many liquid cough medicines. Children find it easier to ingest cough medicines because of flavoring agents that have been added. The sweetness of the vehicle also helps too. As we have learned, this is not the case with other dosage forms, such as solid or semisolid dosage forms.

There are two ways the drug interacts with the vehicle:

• Drug can be dispersed and dissolved throughout the vehicle.
• Drug can be suspended as fine particles throughout the vehicle.

The risk with fine particles is that when the medicine is left stationary in a cupboard or fridge, the fine particles settle to the bottom – forming a kind of cake. That is why it is important to counsel patients to shake the bottle before use.

It re-disperses fine particles throughout the vehicle, allowing for more equal dosing.

Solutions.

Solutions contain solutes (dissolved medicines) within a solvent (liquid vehicle).

The dispersion of the solution is equal. This contrasts with a suspension where it is an unequal distribution of fine particles that are not dissolved.

With solutions, active ingredient is quickly absorbed – whether it be on the skin, through the gastrointestinal tract, or at another site of administration – including the rectal route.

Aqueous solutions have water as the solvent.

Examples include:

• Enemas – solutions introduced to the rectum to empty the bowel.
• Sprays – solutions administered in spray form against the mucous membranes of the nose or throat.
• Injections – sterile solutions that can be administered subcutaneously, intramuscularly, or intravenously.
• Washes – such as mouthwashes, designed to cleanse a part of the body.
• Gargles – designed to treat diseases that impact the throat.
• Irrigations – irrigations are designed to wash the urinary bladder, eyes, or open wounds.
• Douches – administered to a body cavity to disinfect or remove debris.

Thicker aqueous solutions include syrups – which are a mixture of sugar solution and drug.

Non-Aqueous Solutions.

There are two primary kinds of non-aqueous solution:

• Hydro-alcoholic – water and alcohol (elixirs and tinctures)
• Alcoholic – alcohol only

Elixirs are sweetened hydro-alcoholic solutions that taste good, are relatively stable, and which are easy to prepare. The concentration of alcohol is typically between 3 percent and 25 percent.

Tinctures have a much higher alcohol content than elixirs. For example: tincture of iodine. Alcohol levels are typically between 25 percent and 60 percent, though in some cases this is higher.

In the case of tincture of iodine, it has 50 percent alcohol content.

Liquids with Insoluble Matter.

Some liquid solutions have insoluble matter dispersed throughout the vehicle.

Examples include:

• Suspensions
• Emulsions

As the word suggests, suspensions have particles of drug suspended within the vehicle. If the bottle is not shaken before use, the patient receives only a dose of the vehicle – and almost none of the active ingredient.

Drugs that readily dissolve in a vehicle are said to have a high solubility. Suspensions have a low solubility because the drug fails to completely dissolve in the vehicle. Instead, particles remain dispersed. Suspensions are available for oral, topical, and injectable use.

An example of an oral suspension is amoxicillin 125-500mg of drug per 5mL of dose (teaspoon) of medicine.

In the case of injectable suspensions (such as Kenalog-40), they allow for insoluble drugs to be administered to the body in subcutaneous or intramuscular (IM) form. Injectable suspensions are also used for the purposes of depot injections – where the drug is released over a prolonged period (example: Procaine Penicillin G).

Characteristics of the ideal suspension include:

• The ability to evenly – and easily – disperse once shaken.
• Drug should not readily settle to the bottom of the bottle.
• Suspensions should pour easily, and not solidify around the neck of the bottle.
• Suspensions for topical use must have the right consistency. It should not easily run off the skin, nor should it be thick and difficult to apply. Suspensions applied to the skin should dry quickly, not rub off easily, nor have an unpleasant odor or color. Lotions are the most common example of externally applied suspension liquids.

Emulsions are mixtures that contain two kinds of liquid that do not usually mix. For example – oil and water do not mix. But, in an emulsion, it can be created in such a way as to keep the mixture together as much as possible. This is due to the presence of an excipient called an emulsifying agent.

Examples of emulsifying agents include:

• Sodium lauryl sulfate
• Tragacanth
• Polymers known as Spans and Tweens