Pharmacognosy: Drug Adulteration and Drug Evaluation


The therapeutic potential of drugs obtained from natural sources has been recognised by humans for centuries. A rich history of traditional system of medicines gives in-depth knowledge about therapeutic uses of drugs of natural origin. Now a days also drugs of natural origin is popular because of minimum side effects compared to modern system of medicine. However, ensuring the quality, efficacy and safety of drugs of natural origin is a difficult task. In this article we will see drug adulteration and methods os evaluation of drug adulteration. Before that we need to understand about drug adulteration.


Adulteration is defined as substituting original crude drug partially or wholly with other similar looking substances which have less chemical and therapeutic activity. Herbal adulteration is a common malpractice in which intentionally substitution or addition of substandard substances is done to increase the weight or potency of drug or decrease its cost.

Adulteration in simple is the action or process of reducing the quality or value of an article. Adulteration may be intentional or accidental. Adulteration involves different conditions like inferiority, spoilage, deterioration, admixture and substitution.

Unintentional adulteration

Unintentional adulteration occurs due to following reasons.

  • Confusion in names of drug
  • Lack of knowledge about authentic plant
  • Non-availability of authentic drug
  • Similarity in morphology
  • Careless handling

Intentional adulteration

Adulteration using exhausted drug

This type of adulteration is mainly observed in costly drugs. For only that drugs which can be reused after extraction of active chemical constituent of drug. For example, clove and saffron.

Adulteration by using superficially similar but inferior drugs

In this genuine drug is substituted with morphologically similar but less potent substituent. For example, papaya seeds in black pepper and adulteration of cloves by adding mother cloves.

Adulteration by using artificially manufactured substitute

In this the substitute is artificially manufactured which resembles with genuine drug. for example, chicory used in coffee berries and yellow coloured paraffin used as substitute for beeswax.

Adulteration by using substandard commercial verities

In this original drug is substituted with inferior quality drug having same morphological characteristics, chemical constituent or therapeutic activity. For example, substitution of Alexandrian senna with Arabian senna and capsicum minimum is substituted by capsicum annum.

Adulteration by using organic matter obtained from same plant

In this type of adulteration genuine drug is substituted by using one of the parts of same plant. For example, cloves are mixed with cloves stalks and excessive amount of stems in lobelia.

Adulteration by using synthetic chemicals

When synthetic chemical constitute having a chemical constituent of drug is added to the genuine drug. For example, citral is added to lemon oil and benzyl benzoate to balsam of peru.

Drug evaluation

Drug evaluation helps in confirmation of drug identity and determination of quality and purity of drug. It also includes the detection of nature of adulteration. There are several methods available for drug evaluation, like organoleptic (morphological) evaluation, microscopic evaluation, physical evaluation, chemical evaluation and biological evaluation.

Organoleptic (morphological) evaluation

Organoleptic evaluation means study the quality of drugs by using sense organs. It includes the methods of analysis like colour, odour, taste, size, shape and special features like touch and texture. However, the colour, shape and size of drugs described in official books may vary depending on several factors. If plant is grown in favourable conditions, it gives maximum size of leaf, seed and fruits. For example, Alexandrian senna with dog senna or palthe senna identified by this method.

Microscopic evaluation

In this method drug is evaluated by their histological characters. A very thin section of drug is used for histological studies. Microscope helps to enlarge the minute structure of plant. To distinguish the various cellular structure some reagents or stains are used. The techniques like microscopic linear measurement, determination of leaf constant and quantitative microscopy are used in this method of evaluation. Microscopic linear measurements include the study of size of starch grains, length and width of fibres and trichomes, etc. determination of leaf constant includes the study of stomatal number, stomatal index, vein islet number, palisade ratio.

Stomatal number

Stomatal number is the average number of stomata present per sq. mm of epidermis. Stomatal number can be influenced by environmental factors such as light intensity, humidity and carbon dioxide concentration. Stomatal number can vary between different plant species and even among different parts of the same plant. Stomatal number is species specific hence calculating stomatal number helps to confirm the identity of the plant species.

Some examples of stomatal number of upper epidermis of leaf are,

  • Datura stramonium– 87
  • Hyoscyamus niger– 125

Stomatal index

Stomatal index is the ratio of the number of stomata form to the total number of epidermal cells. It is calculated by using following formula,

I = S/(E+S) X 100

Where, I- Stomatal index

S- Number of stomata per unit area

E- Epidermal cells in the same area

Some examples of stomatal index of leaf drug (lower surface) are,

  • Atropa belladonna– 20.2 to 23
  • Indian senna– 17.0 to 20.0

Vein islet number

It is the number of vein islets per sq. mm of leaf surface. Some examples are given below.

  • Digitalis purpurea– 02 to 5.5
  • Digitalis thapsi– 8.5 to 16
  • Cassia acutifolia– 25 to 30

Palisade ratio

It is the average number of palisade cells beneath each epidermal cell. This ratio can be determined with powdered drug. Some examples of palisade ratio are,

  • Atropa belladonna- 06 to 10
  • Digitalis purpurea- 3.7 to 4.2

Quantitative microscopy (lycopodium spore method)

This technique is useful for crude drug identification when chemical and physical methods are inapplicable. Lycopodium spores have characteristic shape, appearance and uniform size. On an average 94000 spores are present in 1mg of powdered lycopodium. Powdered drug can be evaluated by this technique only if it contains:

  • When defined particles which can be counted example, starch grains or pollen grains.
  • The single layered cells or tissues, whose area may be observed under microscope and can be calculated.
  • The object of uniform thickness whose length can be measured under microscope and actual area calculated.

The percentage purity of powdered ginger drug is calculated using following equation,

Percentage purity = (N X W X 94000) / (S X M X P) X 100


  • N- Number of characteristic structures (e.g. Starch grains) in 25 fields
  • W- Weight in mg of lycopodium taken
  • S- Number of lycopodium spores in the same 25 fields
  • M- Weight in mg of sample, calculated on the basis of sample dried at 1050C
  • P- 286000 in case of ginger starch grains powder

Lycopodium spore method is used for evaluation of powdered drugs like clove, ginger, cardamom, nutmeg and umbelliferous fruits.

Physical evaluation

In this method of evaluation physical standards like moisture content, viscosity, melting point, optical rotation, refractive index and solubility are measured.

Moisture content

The moisture content of drug causes decomposition of it either due to chemical change or due to microbial contamination. Hence it is important to determine and control the moisture control of the drug. The moisture content of drug is determined by heating a drug at 1050C in an oven to a constant weight. Some examples of crude drugs with their moisture content limit are,

  • Aloe: Not more than 10.0% w/w
  • Digitalis: Not more than 5.0% w/w


Viscosity is the resistance of a fluid to flow or to change in shape. It is constant at a given temperature. Hence it is used for the identification and standardization of liquid crude drugs. For example,

  • Liquid paraffin: Not less than 64 centistokes
  • Pyroxylin: 1100 to 2450 centistokes

Melting point

The temperature at which solid drug changes into liquid form is known as the melting point. The pure chemical substances have sharp and constant melting point. The crude drugs obtained from animal and plant origin have combinations of chemicals which can be described in specific range of melting point. Some examples are,

  • Colophony: 75-850C
  • Bees wax: 62-650C
  • Hard paraffin: 50-700C

Optical rotation

The optically active substance rotates the plane of polarized light in pure state or in solution. This property is known as optical rotation. Plane of polarized light may rotate in two directions one is right side known as dextrorotatory (+) and another to the left side known as laevorotatory (-). Optical rotation is determined at 250C temperature using sodium lamp as the source of light. Some examples are,

  • Caraway oil: +700 to +800
  • Clove oil: 00 to -1.50
  • Honey: +30 to -150

Refractive index

When a ray of light passes from one medium to another having different density, the path of light changes in second medium, this phenomenon is known as refraction. Refractive index is a ratio of velocity of light in vacuum to the velocity of light in substance. Refractive index is constant for pure liquid at specific incident light, temperature and pressure. For examples,

  • Castor oil: 1.4758 to 1.4798
  • Arachis oil: 1.4678 to 1.4698

Ash content

Ash content is the residue remaining after incineration of drug. It represents inorganic salts which may be naturally occurring in drug or added as adulterant. Different types of ash values are used in evaluation of crude drugs.

  • Total ash value: Many times, a crude drug is adulterated with carbonates, phosphates, silicates and silica. Total ash value is useful to detect these mineral substances.
  • Acid insoluble ash value: It is the value of ash which is insoluble in dilute hydrochloric acid. For example, agar containing not more than acid insoluble ash 1 % w/w.
  • Water soluble ash value: This value is used to detect the presence of material exhausted by water.
  • Sulphated ash value: This value obtained by adding sulphuric acid to get sulphate salts.

Some examples of crude drugs and their total ash contents,

  • Aloes: 05.00 % w/w
  • Ashoka: 11 % w/w
  • Clove: 7 % w/w

Extractive values

The extracts obtained by exhausting crude drugs contain certain chemical constituents. The chemical substances have certain chemical properties based on these various solvents used to determine extractive values. The solvent used according to type of chemical constituents to be analysed.

Water soluble extractivesAloe: Not less than 25% w/w

Ginger: Not less than 10% w/w

Alcohol soluble extractivesRhubarb (50% alcohol): Not less than 30% w/w

Vanilla (45% alcohol): Not less than 12% w/w

Alcohol insoluble extractivesMyrrh: Not more than 70% w/w

Benzoin: Not more than 24% w/w

Ether soluble extractives (non-volatile)Capsicum: Not more than 12% w/w

Cocoa: Not more than 22% w/w

Volatile oil content

Some drugs have efficacy due to their odorous principle, because of presence of volatile oil. These drugs are evaluated on the basis of their volatile oil content. For example,

  • Clove: Not less than 15% w/w
  • Fennel: Not less than 1.4% w/w
  • Dill: Not less than 2.4% w/w

Foreign organic matter

The parts of the organ or organs other than those named in the definition and description of the drug are defined as foreign organic matter. There is a maximum limit of foreign organic matter which is defined in the monograph of crude drug. For example, saffron should not contain more than 2%, satavari should not contain more than 1%, etc.

Chemical evaluation

Chemical evaluation includes qualitative chemical tests, quantitative chemical tests, chemical assays and instrumental analysis. The isolation, purification and identification of active chemical constituents are the methods of chemical evaluation.

Qualitative chemical tests include the identification tests for various phytoconstituents like alkaloids, glycosides, tannins, etc. For example, Vitalis morins reaction for propane alkaloids, iodine for starch, Baudouin’s test for sesame oil in olive oil.

Quantitative chemical tests such as acid value (raisin and balsams), saponification value (balsams), ester value (balsams and volatile oils), acetyl value (volatile oils) are useful for evaluation of drugs by chemical treatment.

Chemical assays include assays for alkaloids, resins, volatile oils, glycosides, vitamins or other chemical constituents. The results obtained from chemical assays confirm the presence of inferior or exhausted drug. For examples the total alkaloid in ipecacuanha, the resin in jalap, etc.

In instrumental analysis various chromatographic and spectroscopic methods are used to analyse the chemical group of crude drugs. Chromatographic methods like paper chromatography, thin layer chromatography (TLC), gas chromatography, high performance liquid chromatography (HPLC), etc. Spectroscopic methods include visible spectroscopy, infrared spectroscopy, mass spectroscopy, etc.

Biological evaluation

This is the advanced technique of drug evaluation in which biological activity, potency and toxicity of a crude drug is evaluated. This method is superior than the physical and chemical method, hence it is mainly used in standardization of the drugs. The potency of a drug is estimated by its effect on living organisms like bacteria, fungi, animal tissue. This technique of potency estimation is known as bioassay. The activity of a drug is measured is represented in International Unit (IU). For example,

  • Digitalis: 1 IU is contained in 76mg of standard preparation
  • A: 1 IU is contained in 0.344 mcg of standard preparation

There are three types of biological assay methods i.e. toxic, symptomatic and tissue method. In case of toxic and symptomatic method animals are used. In the tissue method the effect of drug is studied in-vitro, means activity is observed on isolated organ or tissue.


The issue of adulteration of crude drugs is the significant challenge to the pharmaceutical industry. The adulteration of natural substances compromises with the quality, safety and efficacy of the drug. By using strict quality control measures and advanced testing methods we can combat the issue of drug adulteration.

Frequently asked questions

What is drug adulteration?

Adulteration is the practice of substituting original crude drug partially or wholly with other similar looking substances which have less chemical and therapeutic activity.

What is ash content?

Ash content is the residue remaining after incineration of drug. It represents inorganic salts which may be naturally occurring in drug or added as adulterant.

What are the methods of crude drug evaluation?

There are four methods of crude drug evaluation, morphological (organoleptic) methods, physical methods, chemical methods and biological methods.

What is bioassay?

The technique of estimation of potency of a drug and its effect on living organisms like bacteria, fungi, animal tissue is known as the bioassay.

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