Ashwagandha: what the research does and doesn't show

Ashwagandha's research base doesn't answer many of the questions people search for most: no trial has measured whether it causes weight gain, and there's no dedicated study on using it during pregnancy or while breastfeeding. Here's what the human trials that do exist actually tested — dose, form, population — and where they stop.

What the evidence doesn't support

It doesn't support ashwagandha, in any form or dose, as something that causes or prevents weight gain, eases headaches, or settles constipation or diarrhea — no trial reviewed here measured any of those as an outcome. It doesn't support use during pregnancy or while breastfeeding, or for anyone under 18, at any amount. And it doesn't support a general immune-boosting effect: the immune activity documented for ashwagandha comes from laboratory research, not a human trial measuring immune outcomes.

What the trials found

No trial in this review measured body weight as an outcome. The closest safety data comes from a 2012 open-label study that gave 18 healthy adults (18-30) an 8:1 ashwagandha extract in escalating doses — 750 mg/day, then 1,000 mg/day, then 1,250 mg/day, over 30 days, with no placebo group. Seventeen of 18 tolerated it without incident; one was withdrawn at the lowest dose after developing increased appetite, along with changes in libido and vertigo. That's a single case in an 18-person tolerability study of a concentrated extract, not evidence that ashwagandha changes body weight one way or the other.

Two open-label studies gave infertile men 5 g/day of ashwagandha root powder for three months, using fertile men as an untreated comparison group rather than a placebo. In one, 60 infertile men (heavy smokers, under psychological stress, or with unexplained infertility) showed lower stress markers and improvement on several semen-quality measures compared with their own baseline. In the other, 180 infertile men were tracked with NMR spectroscopy, and several seminal-fluid chemistry markers shifted toward the levels seen in fertile men. Neither study used a placebo, and both were run in men already receiving fertility-clinic care — not a general population, and not the outcome most people searching "ashwagandha for fertility" are asking about.

A separate line of research looked at hormone levels directly. An 8-week, placebo-controlled trial in 50 adults with subclinical hypothyroidism — a mild thyroid imbalance — found that 600 mg/day of ashwagandha root extract shifted TSH, T3 and T4 measurably compared with placebo. That's real, measurable endocrine activity, and it's the reason anyone on thyroid medication should ask a doctor before adding ashwagandha or any new supplement. Separately, a 16-week crossover trial in 57 overweight men aged 40-70 found a low-dose standardized extract (21 mg of withanolide glycosides a day) raised DHEA-S and testosterone compared with placebo, but made no difference to fatigue or vigor — the symptoms the study was actually designed to test.

The clearest signal in ashwagandha's human data isn't a benefit — it's a caution. A 2023 case-report paper described a 36-year-old man who developed liver injury, scored "probable" by a standard causality tool, after taking 1,350 mg of ashwagandha capsules a day for six months; a second case in the same paper, a 30-year-old woman on 450 mg capsules for 45 days, showed a similar pattern. A separate 2024 case report describes a healthy 22-year-old woman who developed liver injury within 30 hours of starting ashwagandha capsules. All three recovered after stopping. Case reports can't establish how often this happens across everyone who takes ashwagandha, but they're real, and multiple similar cases have been published internationally.

Study	Dose & form	Who	Finding
Raut 2012	750-1,250 mg/day, 8:1 extract	18 healthy adults	Well tolerated; 1 withdrawal (appetite, libido, vertigo). No placebo.
Mahdi 2011	5 g/day, root powder	60 infertile men	Lower stress markers, semen-quality changes vs. baseline. No placebo.
Gupta 2013	5 g/day, root powder	180 infertile men	Seminal chemistry shifted toward fertile-control levels. No placebo.
Sharma 2018	600 mg/day, extract	50 adults, subclinical hypothyroidism	TSH/T3/T4 shifted vs. placebo — a caution, not a benefit.
Lopresti 2019	21 mg/day withanolide glycosides	57 overweight men, 40-70	DHEA-S, testosterone up vs. placebo; no change in fatigue or vigor.
Bokan 2023 / Almuzghi 2024	450-1,350 mg/day, capsules	3 adults, case reports	Liver injury, "probable" causality; resolved after stopping.

Questions people ask

Does ashwagandha cause weight gain?

No trial reviewed above tested weight change as an outcome. One person in an 18-person tolerability study developed increased appetite on a concentrated extract — that's a single side effect, not evidence that ashwagandha causes or prevents weight gain in general.

Is ashwagandha safe during pregnancy or while breastfeeding?

Treat it as a hard no rather than a dosing question. Ashwagandha isn't studied for use in pregnancy or while nursing, and it isn't a case of finding the right amount — every source we've checked advises against use in either situation. Ask a doctor before taking any supplement while pregnant or breastfeeding.

Can teenagers or children take ashwagandha, including gummies?

Not without a doctor's guidance. No dosing is established for anyone under 18, and none of the trials above enrolled minors.

Can ashwagandha cause headaches?

None of the trials reviewed here reported headaches as a side effect, but that reflects what those particular studies measured, not proof that it never happens. If a supplement seems to give you a headache, stopping it and checking with a doctor or pharmacist is a reasonable next step.

Can ashwagandha cause constipation or diarrhea?

Neither was tracked directly in the trials above. One safety study reported only mild, unspecified adverse effects in 4 of 50 participants, without breaking out which symptoms occurred.

Does ashwagandha help with bloating or belly fat?

No trial in this review tested ashwagandha for bloating, belly fat or body composition of any kind. The studies that exist measured strength markers, cholesterol, hormones and semen quality — not fat loss.

Does ashwagandha affect fertility?

The only human data comes from two studies in men already receiving fertility-clinic care, using 5 g/day of root powder for three months, with semen-quality changes and lower stress markers compared with baseline. Neither study had a placebo group, and neither addresses what a much smaller amount would do for someone without a fertility diagnosis.

Does ashwagandha affect hormones?

In specific studied populations, yes — a moderate extract dose shifted thyroid hormones in adults with subclinical hypothyroidism, and a low-dose extract raised DHEA-S and testosterone in overweight older men. Neither trial used root powder, and neither is a basis for a hormone-balancing claim in a general population.

Does ashwagandha boost immunity?

Not by any evidence we could find. No human trial in this review tested ashwagandha for immune outcomes. Laboratory research has found immune-stimulating activity, which is actually why people on immunosuppressant medication are told to check with a doctor first — a possible interference, not a basis for an immune-boosting claim.

Does ashwagandha help with pain?

No trial in this review measured pain as an outcome, so there's nothing here to support that use.

When to talk to a doctor

Ongoing symptoms — persistent low mood, disrupted sleep, fatigue, or a fertility concern — are worth raising with a doctor rather than working through on your own with a supplement. Ashwagandha is not a treatment for any of the conditions researchers have studied it against, and nothing above supports using it as a substitute for medical care. Stop taking it and see a doctor if you develop signs of liver trouble — unusual fatigue, dark urine, yellowing skin or eyes, or pain under the right ribs — and check with a doctor first if you're pregnant, nursing, under 18, or take thyroid medication, a sedative, or an immunosuppressant.

Related reading: what is ashwagandha?, how much ashwagandha is actually studied, by form, and who shouldn't take ashwagandha.