Side effects when starting hormone therapy: what is normal in the first few weeks

You start hormone therapy so that you stop waking up drenched at four in the morning and, ten days in, what you notice is tender breasts and tighter trousers. It is one of the most fragile moments of treatment: the benefit has not arrived yet, but the discomfort already has. We are not going to tell you what to do with your treatment here, that is a matter for whoever prescribed it. We are going to explain where the reliable information on adverse effects comes from in Spain, how to interpret it, and why many of the figures circulating online do not survive a fact-check.

Why the product information is the source that matters

When you search for hormone therapy side effects, most results are generic lists mixing studies from twenty years ago, products that are not sold in Spain and personal experiences. The ficha técnica is a different thing altogether: it is the document that the regulator, the AEMPS, approves for each specific medicine, with its specific composition and its specific dose. Its section 4.8 sets out the reported adverse reactions and classifies them by frequency using a standardised scale.

That distinction matters more than it seems. Oestradiol on its own is not the same as oestradiol combined with dydrogesterone, and 1 mg is not the same as 2 mg. That is why talking about the side effects of hormone therapy, in the singular and in the abstract, has little practical use: what applies to your case is in the product information for the medicine you have at home, not in a general article. You can look it up freely on CIMA, the AEMPS public website, by searching for the brand name.

There is one detail almost nobody checks: on CIMA it is not enough to see that a medicine is authorised. You have to look at the marketing status field, because the catalogue changes and there have been supply shortages. A product can be authorised on paper and not be available in your local pharmacy. If you want to understand the treatment as a whole before getting into the early discomfort, menopausal hormone therapy explained: who it is for and its real risks gives you the full picture.

How to read the frequency scale

This is probably the most useful part of the whole article, because it will help you read any patient leaflet for the rest of your life. Spanish product information uses the CIOMS scale, which translates words into probabilities:

• Very common: affects at least 1 in 10 people (≥1/10).
• Common: between 1 in 100 and 1 in 10 (≥1/100 to <1/10).
• Uncommon: between 1 in 1,000 and 1 in 100 (≥1/1,000 to <1/100).

Notice what this implies. Something being listed as common does not mean it will happen to you: it means it happens to fewer than one in ten people. And something being on the list does not mean the medicine definitely causes it either; section 4.8 collects reactions reported during trials and subsequent pharmacovigilance, and some of them simply coincide in time without being a direct cause. Reading a leaflet without this key is a machine for generating anxiety.

The other effect of knowing the scale is that you can tell the noise apart. If someone tells you that a particular symptom is extremely usual at the start, the legitimate question is: usual according to which document and in which frequency category? We are not going to give you the specific frequencies for breast tenderness, bloating, bleeding or headache for Femoston, Meriestra or Estradiol Besins, because doing so rigorously would mean quoting section 4.8 of each ficha técnica word for word, and we are not going to reproduce from memory something you can read directly at source.

What can be said about the first few weeks

Your body has spent months or years adapting to a fall in hormones. Reintroducing hormones is another change, and changes get noticed. The complaints that come up most often at the start of treatment are breast tenderness or soreness, a feeling of bloating, unexpected bleeding or spotting, and headache. They are listed as possible adverse reactions in the product information for hormone therapy medicines, and that is why a professional warns you about them before you begin.

Now the honest part: the idea that early discomfort follows a tidy calendar of four weeks, twelve weeks and beyond is practical clinical consensus, not a figure with a numerical source behind it. It is used because it helps to communicate, not because there is a study setting those thresholds. The same goes for the percentages of irregular bleeding at three, six or twelve months on a continuous combined regimen, which circulate widely and have no public document backing them up. When a piece of data does not exist, saying so is more useful than filling in the gap.

As for the differences between formulations and routes, that is a long conversation full of nuances best taken slowly; in oestradiol and micronised progesterone: what modern hormone therapy looks like we explain how today's components differ from those of earlier generations. And if what you are facing is not yet established menopause but that phase of irregular periods and intermittent symptoms, understanding what perimenopause is and how long it lasts changes how you read what is happening to you.

When to seek advice, and why not to improvise

There is a simple rule that does not depend on any figure: any symptom that frightens you, that is severe, that does not settle or that you were not expecting is a reason to contact whoever prescribed your treatment. That includes bleeding that does not fit what you were told, pain that feels less like discomfort and more like actual pain, and anything that comes on suddenly. You do not need to work out whether it is serious; that is what the consultation is for.

What does not help is adjusting things on your own. Changing the dose, skipping days or stopping abruptly because the first week was uncomfortable are decisions to be made with clinical judgement and your history to hand, not on the basis of an article or a forum. The ficha técnica is an excellent source of information, but it is written for professionals and it is no substitute for someone who knows your case, your background and your other treatments.

One warning worth stating clearly: compounded hormone preparations described as bioidentical, those bespoke formulations offered outside the usual channels, are not evaluated or approved by the AEMPS, the EMA or the FDA. They have no ficha técnica, no section 4.8 and no frequency categories approved by a regulator. That is not bureaucratic small print: it means nobody has verified either what they actually contain or their safety profile.

And if hormone therapy is not an option for you

Not every woman with menopausal symptoms is a candidate for hormone therapy, and not everyone who could take it wants to. The range of alternatives has shifted in recent years: there are non-hormonal options aimed specifically at hot flushes, as we explain in the article on fezolinetant and the non-hormonal route for hot flushes, and then there is the far noisier territory of supplements, where it pays to separate the wheat from the chaff: we go through it in phytoestrogens and isoflavones for menopause: what works and what does not.

In summary

The first few weeks of hormone therapy tend to be the most confusing, and the most useful tool you have is knowing how to read your medicine's product information and its frequency scale: that saves you half the anxiety and all the online noise. What no article can give you is judgement about your particular case, and that does exist in Spain today: menopause treatment with a prior assessment, prescription when appropriate by a registered doctor, and ongoing follow-up, which is exactly where these first-few-weeks questions get resolved. You can start by checking whether you are eligible in a few minutes.