Forskolin (Coleus forskohlii): what the research does and doesn't show

Forskolin's two human trials didn't find or measure weight gain, and neither reported diarrhea as a side effect — but neither trial specifically tracked digestive symptoms as its own outcome, so that's a gap in what was measured, not a clean "no" on the question.

What the evidence doesn't support

It doesn't support the idea that forskolin causes weight gain — neither of the two existing trials found that, and one found a trend toward less weight gain than placebo. It doesn't support or rule out a link to diarrhea specifically: neither trial's published results break out digestive symptoms as an individually tracked outcome. And it doesn't support a specific dose "for effective weight management" — no trial has compared different doses of forskolin against each other, and the two trials that do exist, using the same dose, disagreed on whether forskolin did anything for weight at all.

What the trials found

A 2005 trial in 30 overweight or obese men gave 250 mg of a 10%-standardized forskolin extract twice a day for 12 weeks. Its published results cover body composition and hormone changes — lower body fat percentage, higher bone mass and free testosterone versus placebo — without a separate adverse-event or side-effect tally distinct from those outcomes (PMID 16129715). A separate 2005 trial in mildly overweight women used the identical extract and dose for the same 12 weeks, and tracked side effects weekly. It found no clinically significant differences from placebo across a wide panel of safety bloodwork, heart rate, blood pressure, or the weekly side-effect reports, and the forskolin group actually tended to gain less body mass than placebo over the study, not more (PMID 18500958). Neither trial's published record specifically calls out diarrhea as a tracked symptom.

A 2012 pharmacology review, based on forskolin's mechanism rather than a dedicated side-effect trial, flags increased stomach acid as a possible effect and a reason for caution around ulcers (PMID 23023353) — a different, mechanism-based concern from ordinary diarrhea, and not itself a report of digestive side effects in a human trial.

Questions people ask

How much forskolin is used in the weight-management research?

Both existing human trials used the same amount: 250 mg of a 10% extract, twice a day (500 mg extract/day, about 50 mg of forskolin itself). No trial has tested a different dose, and the two trials at this dose reached opposite conclusions — see does forskolin help with weight loss? for the full results.

Can forskolin cause weight gain?

No. Neither trial found forskolin associated with weight gain; the women's trial specifically found a trend toward less weight gain than placebo, though that wasn't the study's main statistically significant finding.

Can forskolin cause diarrhea?

Neither of the two existing human trials specifically reported diarrhea as a tracked side effect. That's a gap in what was measured and published, not a confirmed absence of the symptom in everyone who takes it.

When to talk to a doctor

Forskolin is not a treatment for weight management, and nothing here is a substitute for medical guidance. If you develop a new or persistent digestive symptom, or have a history of ulcers or another GI condition, talk to a doctor before taking forskolin, given the mechanism-based caution above. If you're pregnant or nursing, or take a prescription medication, ask a doctor first — see forskolin and medications: what to check.

Related reading: forskolin side effects, forskolin benefits: what the research does and doesn't show, and who should not take forskolin?