Dangerous Drugs in Pregnancy (Teratogenic) List, Types

Pregnancy is often seen as a very delicate time in a woman’s life. The concern is more often for the fetus than the mother, as it is fragile and the risk of miscarriage or developmental abnormalities are high in the early stages of pregnancy. Plenty of rest, a balanced diet and healthy lifestyle will largely suffice in ensuring that both mother and child are not at risk. It is well known that tobacco or alcohol consumption, exposure to pesticides and even certain skin applications should be strictly avoided during pregnancy. However, many women are not as cautious when it comes to medication – both OTC (over-the-counter) and prescription drugs.

Should drugs be used in pregnancy?

Firstly it is important to understand that not all pharmaceutical drugs are dangerous in pregnancy. There is no need to avoid necessary medication if your doctor and pharmacist deem it safe to use during pregnancy. However, it is advisable to avoid any and all substances, be it pharmaceutical or otherwise, if it is not necessary.

Secondly, it is important to research each medication that you may use. The package insert provides crucial information in this regard. Coupled with the advice of your doctor and pharmacist, you should make an informed decision about what to use and what to avoid. Many drugs which are safe for use during pregnancy.

It is important to remember that some drugs are essential. Like progesterone tablets in the first trimester where there is persistent vaginal bleeding or spotting can help save the pregnancy. All drugs should not be portrayed in the same light – in fact some nutritional substances (vitamin and mineral supplements) can be more dangerous than certain drugs, especially when it is used in large doses.

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What is a teratogenic substance?

A teratogenic substance, or teratogen, are chemicals that affects the normal growth and development of a fetus. These abnormalities or malformation  can even affect the viability of the pregnancy. It does not only apply to pharmaceutical drugs. Any substance that is dangerous to the fetus falls into this category, including alcohol, tobacco, illicit substances (street drugs), herbal medicines and certain nutritional substances. Some pesticides and certain food additives are also potentially teratogenic substances.

Teratogenic agents can also include certain infectious agents like viruses that can affect the fetus, physical factors like abnormal shape or size of the uterus that may impede fetal growth, and electromagnetic waves like ionizing radiation. However, in terms of teratogenic substances it specifically refers to chemicals. The focus of this article is on pharmaceutical drugs which are potential teratogens. Some are well known as potential teratogens, others have not been conclusively identified as such.

Types of Drugs

Apart from chemotherapeutic agents (cancer drugs), teratogenic substances cannot be solely classified by the type of drug. For example, tetracycline is a well known group of antibiotic and teratogen but this does not mean that all antibiotics need to be avoided in pregnancy. For this reason individual drugs are classified according to categories (A, B, C, D, X), with category A being considered absolutely safe for use in pregnancy and category X being the most dangerous.  Vaccines are another concern for many pregnant women. Live vaccines are generally avoided but some vaccines are administered if absolutely necessary.


The Food and Drug Administration (FDA) has assigned pregnancy categories which are used to determine whether a drug is safe or not. It also applies to breastfeeding mothers as substances can be passed on to the newborn during lactation.

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Category A

Adequate and well-controlled studies have failed to demonstrate a risk to the fetus in the first trimester of pregnancy (and there is no evidence of risk in later trimesters).

This simply means that the research studies and clinical trials conducted on these substances have shown that there is no risk to the fetus within the first trimester of pregnancy. It is therefore deemed safe to use these substances.

ADVICE: Use freely but within the prescribed dosage.

Category B

Animal reproduction studies have failed to demonstrate a risk to the fetus and there are no adequate and well-controlled studies in pregnant women.

This means that animal studies have not as yet shown that the substance in this category may be a risk to the fetus. However, there were no studies specifically performed on pregnant women to verify the safety of these substances in terms of its teratogenicity.

ADVICE: Use but with caution (moderate dose and duration).

Category C

Animal reproduction studies have shown an adverse effect on the fetus and there are no adequate and well-controlled studies in humans, but potential benefits may warrant the use of the drug in pregnant women despite potential risks.

This means that there is a possible risk to the fetus by using these substances, although this has not been verified in proper human studies. However, the substance should be used if the potential benefits outweigh the risks.

ADVICE: Use only if necessary, according to a doctor’s instruction.

Category D

There is positive evidence of human fetal risk based on adverse reaction data from investigational or marketing experience or studies in humans, but potential benefits may warrant use of the drug in pregnant women despite potential risks.

This means that there is conclusive evidence of a risk to the fetus. Risk does not mean that there is danger in every instance. Despite this risk, the substance in this category may still be used if it is absolutely necessary, where the potential benefits outweigh the potential risks.

ADVICE: Avoid as far as possible, unless a doctor instructs otherwise.

Category X

Studies in animals or humans have demonstrated fetal abnormalities and/or there is positive evidence of human fetal risk based on adverse reaction data from investigational or marketing experience, and the risks involved in use of the drug in pregnant women clearly outweigh potential benefits.

This means that there is significant and conclusive risk to the fetus by using the substances in this category. The potential benefits cannot justify use of this drug due to the known risks to the fetus.

ADVICE: Avoid altogether.

List of Dangerous Drugs

There are a number of different drugs that are not deemed suitable for use in pregnancy. New drugs are constantly being launched and older drugs may still be in use in certain nations. Therefore it is difficult to formulate a complete list of these drugs. Here are five drugs (or types of drugs) that should be avoided in pregnancy, specifically within the first trimester.


Tetracycline is a type of antibiotic. It is can cause a yellow staining of the teeth and reduce growth of long bones. Some types of antibiotics may not hold the same risk and can therefore be used safely during pregnancy.


Phenytoin is an anticonvulsant. It can retard fetal growth within the uterus, cause an abnormally small head, lead to mental retardation and affect normal development of facial features.

Anti-neoplastic agents

Anti-neoplastic agents are cancer drugs (chemotherapeutic drugs). It inhibits rapid cell division in the body and should only be considered for use in the third trimester of pregnancy.

Retinoic acid

Retinoic acid is a vitamin A derivative. The extreme teratogenicity associated with retinoic acid applies to other vitamin A derivatives as well. It may cause neural tube defects, cleft palate, other deformities of the facial bones and cranium, as well as absence of the thymus.


Thalidomide is an immune-modulating drug, that was used widely as a tranquilizer in the 1950s. It can cause abnormalities or even the absence of certain appendages, congenital heart disease and congenital urinary malformations.




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