Oral supplementation of carnitine for 6 months appears to be roughly as effective as surgery for improving sperm counts as surgery for varicoceleoctomy in the trials conducted so far - at least at the lower grades of varicocele (there is no effect in the highest grades 4 and 5). The results show increased sperm count and motility, and raised testosterone.

It is unclear whether carnitine treatment actually improved the varicocele, or just reduced the consequences of it (e.g. oxidative stress)

Human Experimental Trials

Quasi experimental trial (without randomization): Administration of oral L-Carnitine is as effective as varicocelectomy in grade 2 varicocele in improving semen parameters
Inclusion criteria: was infertile married men with varicocele grade II and higher.
Participants: 62 patients, 31 in surgery group, 31 in carnitine group. 19 to 41 years with a mean age of 29.06 years.
Duration: 6 months.
Study Design: Non-randomised trial. Patients at clinic were asked if they wanted pills (carnitine not mentioned) or varicocelectomy.
Dose: 250 mg L-carnitine four times a day

RCT: Carnitine + Acetyl-L-Carnitine boosts sperm count in men and improves pregnancy rates. Effects boosted further by cinnoxicam suppository
Duration: 6 months
Participants 325 patients, divided into 3 groups.
Groups: (1) Placebo, (2) Oral L-carnitine (2 g/d) + acetyl-l-carnitine (1 g/d), (3) l-carnitine/acetyl-l-carnitine + 1 × 30-mg cinnoxicam (a nonsteroidal anti-inflammatory drug [NSAID]) suppository every 4 days
Results: There were no changes in group 1, in group 2 there were improvements in sperm count and motility, but not significantly so. However, there was a significant (and relevant) increase in pregnancy rates. The addition of cinnoxicam boosted all results, making the sperm results significant.
Unfortunately, the higher the degree of varicocele is the lower the benefit form therapy. No drug proved active in grade V varicoceles.