Spirulina

What it is

Spirulina is a microalga, Arthrospira, cultivated for its protein, vitamins and minerals [1][2]. It is sold as powder, tablets and capsules, and trials have also added it to dressings and desserts [3][4][5].

How it might work

Proposed: spirulina contains peptides that may block angiotensin-converting enzyme (ACE), which raises blood pressure. This was seen in lab tests of the raw biomass, but not after the spirulina was built into a yogurt-like dessert [5]. Proposed for cholesterol: spirulina may reduce how much cholesterol the gut absorbs, but in a 17-day human study, markers of absorption did not change [6]. Proposed for weight: reduced appetite; in a 12-week trial, appetite fell more with spirulina than with placebo [3].

What the research says

Weight. In a network analysis of 111 trials of 18 supplements, spirulina lowered body weight by 1.77 kg compared with placebo [7]. A separate meta-analysis found 1.85 kg less weight than controls, with wide variation between trials [8]. A meta-analysis of algae supplements found 0.78 kg less weight, with low-certainty evidence [9]. In a 12-week trial of 38 adults who finished, spirulina with a calorie-restricted diet led to more weight loss than placebo with the same diet [10]. In a 12-day study of 40 wrestlers, body mass fell by about 3 kg in both groups, and only body fat fell further with spirulina [11].

Blood pressure. Three meta-analyses found systolic pressure about 4 to 5 mmHg lower and diastolic about 3 to 7 mmHg lower with spirulina [1][12][13]. A 2025 review of 29 trials of edible algae found spirulina the most effective alga, and more than 3 g a day for at least 12 weeks gave the clearest results [14]. Single trials are less consistent: an 8-week spirulina dressing lowered blood pressure against placebo in 41 adults with hypertension [4]; an 8-week spirulina-enriched dessert did not change it in 64 overweight adults [5]; a 6-month trial in 120 healthy older adults found no overall difference [15]. A 2025 meta-analysis of 9 trials reported only a small diastolic effect, which its authors called of uncertain clinical significance [16].

Cholesterol. Results are mixed. A 2016 meta-analysis of 7 trials reported LDL about 41 mg/dL lower [17]. A 2019 meta-analysis in people with metabolic syndrome found about 19 mg/dL lower [18]. A 2023 meta-analysis of 20 trials (1,076 people) reported lower LDL and triglycerides [19]. A 2025 review of 35 trials found about 8 mg/dL lower and rated the LDL evidence low quality [13]. A 2025 review of 9 trials found no significant change in lipids [16]. In a 17-day trial of 35 healthy adults, lipids and absorption markers did not change [6].

Doses used in studies

Doses in the trials cited here ranged from 1 g to 4.5 g a day, lasting from 12 days to 6 months [3][20][15][11]. Two trials used 2 g a day for 8 to 12 weeks [4][10]. A 2025 review suggested more than 3 g a day for at least 12 weeks for blood pressure [14]. These are trial doses, not recommendations.

Safety and interactions

Side effects. A 3-month trial in 60 adults with type 2 diabetes on metformin reported no adverse events [21]. Trials of up to 6 months are the longest we found, so long-term effects are not established [15].

Medicine interactions. A 2-week study of a high-dose phycocyanin extract (2.3 g a day, 24 people) found no change in clotting or platelet markers [22]. We found no other interaction studies.

Upper limits. We found no upper safe limit or toxic dose in the studies read.

Contamination. Some blue-green algae products contain microcystins, which damage the liver. In an Italian study of 17 brands, spirulina-only products were free of them, while Aphanizomenon-based products were contaminated [23]. In data from 95 French spirulina producers, microcystin levels stayed within the safe regulatory level [24].

Pregnancy, breastfeeding, kidney and bleeding disorders. We found no studies of these groups.

Caveats and open questions

Most trials enrolled a few dozen people and ran for 8 to 12 weeks; the largest single trial read enrolled 120 people for 6 months [10][3][15]. Results conflict: the 2025 lipid review and a 2026 blood pressure trial found no effect [16][5]. The trials read measured blood pressure, cholesterol and weight, not heart attacks or strokes [13]. A 2025 review of overweight adults found the overall effect on body composition and blood sugar limited [25]. Two studies had industry ties: one 2025 review had authors employed by Sea4Us [16], and a 2025 trial had an author employed by the supplier [15].