Drug Coating | Active Ingredient Coating On Tablets

Every one of us is familiar with film common coating types like enteric coating & tablet sugar coating but very few of us are familiar with drug coating which is also known as API Coating or active pharmaceutical ingredient coating over Tablets.In this article, we will discuss & learn the following,

  • What Is Drug Coating?
  • When or Why To Use Drug Coating
  • Steps Of Drug Coating
  • Monitoring of API Coating 
  • Preparation of drug coating suspension.
  • Procedure Of Drug Coating
  • Limitations of Drug Coating 


Drug Coating



What Is Drug Coating?

Drug Coating is also known as Active Pharmaceutical Ingredient Coating or API tablet coating, as the name indicates it is the type of tablet coating in which we apply a layer of active pharmaceutical ingredient over the surface of the tablet along with other tablet coating excipients.

When To Use Drug Coating

As we know that coating is usually done to mask the taste, protect the product from environmental effects or alter the drug release but the purpose of drug coating is different & it is used for the following causes,

  • To Obtain 2 different drug release patterns in the same tablet.
  • When 2 APIs are not Compatible With Each other.

To Obtain 2 Drug Release Patterns in the same tablet.

If we have two active pharmaceutical ingredients & want to make one API sustain release & other active pharmaceutical ingredients as immediate release then it can be done by following 2 methods,

  • Layered Tablets
  • Drug Coating 

Layered Tablets

Bi later & trilayer tablets are available in the market which is used for the above-mentioned purposes to obtain sustained release & immediate release effects in the same Tablet where one layer is semi-compressed first, containing one API e.g one layer manufactured by wet granulation containing HPMC to make API sustain release & then 2nd layer is added over it & both layers are compressed by applying main pressure in which 2nd layer is of immediate release & a first layer of the extended-release drug.

Limitations Of Layered Tablets 

Overall tablet weight is the main concern due to which usually the layer tablet method is not implemented for all products & we have to go towards drug coating.

  • Suppose we want to manufacture a drug product containing Metformin HCl as sustained release & sitagliptin phosphate as immediate release & the strength claimed is Metformin HCl 1000mg & sitagliptin phosphate 50 mg.
  • We can prepare sustain release granules of Metformin HCl using HPMC grades by wet Granulation method & suppose its Compression weight is 1500 mg per tablet.
  • If it is an oblong tablet its dimensions will be very large like 12×24 mm.
  • If we manufacture sitagliptin 50 mg layer by wet granulation or direct compression methods & its compression weight is kept low like 200 mg, it will be a very thin layer which may be difficult to maintain over 12×24 mm long tablets & difficult to remove for weight monitoring.
  • If we prepare a sitagliptin layer with a compression weight of 800 mg it will be easily formed & separated but the overall weight of the compressed tablet will be very high like 2300 mg which is not feasible for patients to swallow such a large-size tablet.

Drug Coating 

To overcome the above-mentioned issue we use drug coating where we prepare sustain-release granules of Metformin HCl by wet granulation method & compress the granules to obtain sustain-release tablets of Metformin HCl at compression weight 1500 mg.

The sitagliptin phosphate layer is added in coating suspension which is immediate release & it is sprayed over the surface of the tablet to deposit 50 mg of sitagliptin phosphate on each tablet & usually, weight gain of 300 mg with other excipients is achieved after drug coating so total weight after drug coating will 1800 mg and it is less than 2300  mg when it is compressed in form of bilayer tablets.

Steps Of Drug Coating

Drug Coating is usually done in the following steps depending on the core tablet properties & required results.

  • Protective Layer
  • Drug Layer
  • Taste Masking Layer 

Protective Layer

If we are doing the drug coating for the case where our 2 APIs are not compatible with each other we can apply a coat of inert polymer over the surface of core tablets before applying a drug coat to make a barrier between ttwo Active Pharmaceutical Ingredients.

A protective coat can also be used in the case where our core tablet surface may be affected by the damaging effects of the spray in the long run so we give extra protection to the tablets by a protective or sealing cover.

Drug Layer

It is the main step where we apply coating suspension containing API over the tablet surface. It is the most critical Process as the drug deposition on each tablet must be uniform to get content uniformity & uniform results of assay & dissolution. 

So critical monitoring of drug layer deposition is required.

Taste Masking Layer 

The taste masking layer is applied in  those cases where our API has a bitter taste, so to mask the bitter taste we apply a thin coat of inert polymer over the drug layer surface to mask its taste. This step is same as the simple film coating process.

Monitoring Of API Coating 

Drug Coating or API coating is not as simple as film coating because here our API is directly deposited over the tablets surface so critical monitoring of weight gain is done after validated time intervals to ensure uniform weight gain after each monitoring interval.


We must have to validate our monitoring intervals & weight gain. We can monitor weight gain after every 30 minutes 45 minutes or 60 minutes.

Suppose we monitor weight gain after every 45 minutes, the weight gain must remain uniform after every 45 minutes.

Suppose we check at the following intervals & weight gain is given below,

  •  At 9:00 am  Weight Gain is 7 mg
  •  At  9:45 am  Weight Gain is 9 mg
  •  At 10:30 am Weight Gain is 8 mg

It is the reflection of uniform weight gain.

If we have the following pattern, it will be the reflection of non-uniform weight gain.

  • At 9:00 am  Weight Gain is 7 mg
  •  At  9:45 am  Weight Gain is 3 mg
  •  At 10:30 am Weight Gain is 14 mg

If we observe non-uniform weight gain it reflects that our coating parameters are not accurate so adjustment & critical monitoring of all coating process parameters is also very important.

Preparation Of Drug Coating Suspension.

As we know that the normal coating suspension has the following components,

  • Solvent
  • Polymer
  • Plasticizer
  • Opaquants
  • Colors 

In the case of drug coating, we have one additional component which is the drug or API.

The suspension is prepared by adding & mixing all the ingredients in the solvent according to the formulation & API is also added to the solvent to get the coating suspension containing API or Drug.

Procedure Of Drug Coating

First of all, add core tablets to the coating pan & rotate to remove the powder then apply heat to warm the tablets & start spraying process of the coating suspension.

First of all, apply a protective layer suspension & note the weight gain at the end of the suspension.

When the protective layer suspension is finished then apply the drug coating layer & monitor the weight gain after specified time intervals.

After drug coating, apply a taste masking coat if required and monitor weight gain at the end of the spraying process.

Bring the tablets temperature down & off load the tablets from the coating pan.

Limitations of Drug Coating 

As we discussed earlier drug Coating is a very critical process so for its process, a trained staff is required under the strict supervision of a qualified person.

For drug coating advanced type of coating machines are required where the opening & closing of the exhaust can be controlled by HMI along with other parameters.

Drug Coating can’t be performed on conventional coating pans or on coating machines that have a poor exhaust control system.

Also Read 

  1. Basis of Tablet Coating.
  2. Components Of Tablet Coating Suspension
  3. Technical Aspects of Tablet Coating Process.
  4. Weight Gain Defect In Coated Tablets