Hair loss after an illness: what is known

After an illness with a high fever, COVID-19, pneumonia or a hospital stay, many people notice, two to four months later, that hair starts to fall out in large amounts. By the time the hair loss appears, the illness has passed and so it is hard to make the connection. In a large study of people hospitalised with COVID-19, more than one in five reported hair loss six months later.

In most cases it is a telogen effluvium: the body, under stress, "pauses" many hairs at the same time, which fall out weeks later. It is frightening, but as a rule temporary, and the hair grows back.

In this guide we explain why it happens, which illnesses and medicines are most associated with it, how long it lasts, what helps recovery and which signs indicate that there is another cause behind it.

Why an illness makes hair fall out

The mechanism: telogen effluvium

At any one time, about 85% to 90% of hairs are growing and 10% to 15% are resting. When the body goes through a "shock", such as an acute illness, high fever, surgery or a hospital stay, up to 70% of growing hairs can move into the resting phase at the same time (DermNet).

The hairs stay anchored in the follicle for about 3 months and then fall out, pushed by new ones. That is why hair loss usually appears 2 to 4 months after the illness.

Which factors of the illness contribute

• High fever: it is one of the classic triggers.
• Generalised inflammation: the inflammatory substances released during an infection also affect the follicle.
• Loss of appetite and weight: during illness you eat little, and hair misses the protein and nutrients.
• Physical and emotional stress: hospital stays, intensive care, anxiety.
• Medicines: some medicines used during or after illness can also contribute.
• Immobility and prolonged rest, accompanied by muscle loss.

Effluvium does not destroy the follicles. That is why, in the great majority of cases, hair grows back. We explain the hair cycle in Hair loss: why it happens and what to do.

Which illnesses make hair fall out

COVID-19

COVID-19 made post-illness effluvium very visible. In a study published in The Lancet of 1,733 people hospitalised in Wuhan, followed for about 6 months after discharge, 21.7% reported hair loss. The most common symptoms were tiredness or muscle weakness (62.7%), difficulty sleeping (26.4%) and hair loss (Huang et al., 2021).

In a case series and literature review, hair loss began on average 73 to 87 days after infection, about 2.5 to 3 months. Hair loss can also appear after mild forms, especially if there was fever.

Other infections with fever

Influenza, pneumonia, dengue, mononucleosis, severe urinary infections and other infections with high fever can trigger the same process. Fever and inflammation matter more than the type of microbe.

Hospital stays, operations and intensive care

Major surgery, prolonged anaesthesia, blood loss and stays in intensive care are frequent triggers. To these are added immobility, reduced food intake and medicines. We explain in Surgery, anaesthesia and hair: what to expect.

Chronic illnesses

Some chronic illnesses cause diffuse hair loss, continuous or in episodes:

• thyroid disease, hypothyroidism or hyperthyroidism;
• autoimmune diseases, such as lupus;
• inflammatory bowel diseases and coeliac disease, because of poor nutrient absorption;
• chronic kidney and liver disease;
• poorly controlled diabetes;
• iron-deficiency anaemia.

In these cases, treating the underlying condition is the first step towards hair recovery.

Medicines that can make hair fall out

Some medicines can cause telogen effluvium. A review published in the journal Cutis recommends looking for medicines started 1 to 6 months before the onset of hair loss (Zhang et al., 2024).

Source: Zhang et al., 2024, and clinical synthesis.

Chemotherapy: a different case

Chemotherapy causes a different type of hair loss, anagen effluvium: the drugs attack rapidly dividing cells, including those of the growing follicle, and hair falls out much earlier and in greater amounts.

• Hair loss usually begins 1 to 3 weeks after the first cycle and increases with subsequent cycles.
• Hair usually grows back in the 6 months following the end of treatment, sometimes with a different texture or colour.
• In some people, especially with taxanes, regrowth is incomplete after 6 months; this is called persistent alopecia.
• Scalp cooling during chemotherapy reduced hair loss: in a review of trials, the occurrence of alopecia fell by about 2.7 to 3.9 times (Peera et al., 2024). It should be discussed with the oncology team.

When it starts and how long it lasts

In acute effluvium, hair loss usually normalises after 6 to 9 months. When it lasts longer than 6 months, it is called chronic effluvium, which deserves investigation (DermNet).

A reassuring sign: when hair loss starts to slow, short, spiky hairs appear along the front hairline and on top. It is new hair growing.

What helps recovery

1. Get back to eating well

During illness, you eat little. In recovery, the most important thing is to return to eating regularly and with protein at every meal. See Recovering calmly: what helps the body and the hair and Protein, iron and zinc: what your hair needs.

2. Correct deficiencies

After a prolonged illness or a hospital stay, it is worth checking:

• full blood count and ferritin, especially if there has been blood loss or anaemia;
• TSH, because some infections and medicines affect the thyroid;
• vitamin D and B12, depending on the history.

See Blood tests for hair loss: what to ask for.

3. Review medicines with your doctor

If you started new medicines in the previous months, talk to your doctor. Sometimes there are alternatives; often the medicine should be kept.

4. Give it time and treat your hair gently

• Wash as normal: washing does not increase hair loss.
• Loose hairstyles and less heat.
• Avoid harsh chemicals until hair loss settles.
• Photograph the top and the parting to compare after 3 months.

5. Sleep and manage stress

Recovering from an illness is tiring. Sleep and stress management help the body and avoid new triggers. See Stress and hair loss: what is known.

6. Medical treatment, if needed

In isolated acute effluvium, treatment is generally not needed. If hair loss goes on or reveals an androgenetic alopecia, the doctor may propose treatments such as minoxidil.

Signs that it is not just an effluvium

Post-illness effluvium is diffuse, across the whole scalp, and temporary. These signs suggest another cause or an associated cause:

In these cases, an assessment with trichoscopy helps to clarify. See Trichoscopy: the examination that shows the scalp.

When to seek a medical assessment

• The hair loss lasts more than 3 to 6 months or there are no signs of regrowth.
• You noticed one of the signs in the previous section.
• You started new medicines in the previous months and want to understand whether they are related.
• You have tiredness, pallor or other symptoms that persist after the illness.
• You are having or about to have chemotherapy and want to know how to protect your hair: also talk to the oncology team.

At Luxmed, the Hair Medicine consultation assesses the type of hair loss with trichoscopy and, when indicated, blood tests, to confirm whether it is a post-illness effluvium or another cause, and follows your recovery.

This article is for information only and does not replace a medical consultation. Do not stop any medicine without medical advice.

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