Ashwagandha is one of the most popular herbal supplements on the market right now, and it has been shown to have quite a lot of benefits, with reasonable evidence to support it's use as a sleep and stress aid, right through to it's use an exercise supplement. However, that's not to say that there aren't any draw backs which brings us to the question of what are the side effects of ashwagandha?
We should start by saying we're going to need to break this down into common side effects of ashwagandha and some of the rarer cases. It is also worth noting that a lot of supplements have examples of extreme side effects that really aren't something to worry about all to much when we're talking about 1 in a million reaction cases.
There are also some relatively common ashwagandha interactions with medications, this shouldn't be too surprising when you consider the uses of ashwagandha, for example, if ashwagandha is being used as a sleep aid, it stands to reason that it's going to interact with sleep aids. In this sense potential Ashwagandha side effects are directly related to it's benefits.
Then there is of course the issue of simply taking too much ashwagandha, 600mg is really the top end that you'll want to be at, seeing as that's been shown to be safe in multiple clinical trials. 
Lastly for the intro, Ashwagandha is considered safe, and whilst nausea is a relatively common side effect of the herb (more than many others), it rarely has issues that are much beyond this. 
The Most Common Side Effects of Ashwagandha
Gastrointestinal Upset - Taking large doses of ashwagandha may cause digestive upset, including diarrhea, nausea, and vomiting. This is believed to be due to the direct irritation of the intestinal mucosa.  It is important to follow the recommended dosage guidelines and to stop taking ashwagandha if you experience any gastrointestinal discomfort.
Thyroid Condition Interactions - Ashwagandha has been shown to have an impact on thyroid hormone levels. It may increase thyroid hormone production, which can be problematic for individuals with hyperthyroidism or those taking thyroid hormone medications.  If you have a thyroid condition or are taking thyroid medication, it is important to discuss the use of ashwagandha with your healthcare professional.
Drowsiness - Ashwagandha is known to promote relaxation and support a healthy sleep cycle. While this can be beneficial for individuals struggling with sleep issues, it may cause drowsiness, especially when taken in higher doses or in combination with other sleep-promoting supplements or medications. 
Some of the same studies also showed a reduction in appetite can coincide with drowsiness.
Common Interactions With Ashwagandha
Ashwagandha may interact with certain medications, including sedatives, immunosuppressants, thyroid medications, anticonvulsants, anti-diabetic drugs, and anti-hypertensive drugs. It is important to consult with a healthcare professional before using ashwagandha if you are taking any medications to avoid potential interactions.
As we mentioned in the intro, considering ashwagandha is used as a sedative, is known to effect hormones (and is generally used to raise testosteorne in males or lower cortisol levels) it's intuative that it could impact these sorts of medications. The same applies to the fact it has been shown to lower blood sugar in multiple trials. 
It's also worth noting that people who have stomach ulcers should also not take ashwagandha.
Serious But Rare Ashwagandha Side Effects
Allergic Reactions - Ashwagandha belongs to the nightshade family of plants, which also includes potatoes, tomatoes, peppers, and eggplant. People with allergies or sensitivities to nightshades may experience allergic reactions when using ashwagandha. It is important to discontinue use if you experience any signs of an allergic reaction, such as itching, rash, or swelling.
Liver Toxicity - There are some rare cases that have been reported of ashwagandha causing liver toxicity, however, in most of these cases the users had taken a significantly larger dose of ashwagandha than is recommended. 
Unknown Side Effects of Ashwagandha
Pregnancy and Breastfeeding - There is limited research on the safety of ashwagandha during pregnancy and breastfeeding. It is recommended to avoid using ashwagandha during these times to ensure the safety of both the mother and the baby. If you are pregnant or breastfeeding and considering using ashwagandha, it is best to consult with a healthcare professional.
How to Use Ashwagandha Safely
To use ashwagandha safely and minimize the risk of side effects, it is important to follow these guidelines:
- Start with a low dose: Begin with the lowest recommended dose and gradually increase if necessary. This is generally around 300mg which is the most studied dose of ashwagandha for sleep and stress.
- Follow recommended dosage guidelines: Stick to the recommended dosage provided by the manufacturer or as advised by a healthcare professional.
- Consult with a healthcare professional: If you have any underlying health conditions, are taking medications, or are pregnant or breastfeeding, it is important to seek guidance from a healthcare professional before using ashwagandha.
- Choose high-quality products: Look for reputable brands that adhere to quality standards and have third-party testing to ensure the purity and potency of their products.
1 - https://www.nccih.nih.gov/health/ashwagandha
2 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10147008/
3 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3103490/
4 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9035336/
5 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3573577/
6 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3252722/
7 - https://www.ncbi.nlm.nih.gov/books/NBK548536/